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How to Build a Sudoku Solver Android App?

Last Updated : 24 Apr, 2025
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In this article, we are going to make a sudoku solver Android app in Kotlin and xml. The main focus of the app will be on the sudoku-solving algorithm so the the design part has been hardcoded. A sample video is given below to get an idea about what we are going to do in this article.

Step by Step Implementation

Step 1: Create a New Project in Android Studio

To create a new project in Android Studio please refer to How to Create/Start a New Project in Android Studio. Note that select Kotlin as the programming language.

  • Now we will create the layout of the sudoku solver using Table Layout, you just need to create one row and copy paste that 9 times and give the ids as shown.
  • Although you can reduce this redundancy using recyclerview but to keep it simple I have Hardcoded the table.

Add viewBinding in builde.gradle(app:Module) inside android{} and sync the files.

Kotlin 
viewBinding{   enabled = true }

Step 2: Working with the activity_main.xml file

Navigate to the app > res > layout > activity_main.xml and add the below code to that file. Below is the code for the activity_main.xml file. Comments are added inside the code to understand the code in more detail.

XML 
<?xml version="1.0" encoding="utf-8"?> <LinearLayout      xmlns:android="http://schemas.android.com/apk/res/android"     xmlns:tools="http://schemas.android.com/tools"     android:layout_width="match_parent"     android:layout_height="match_parent"     android:orientation="vertical"     android:padding="15dp"     tools:context=".MainActivity">      <TableLayout         android:layout_width="match_parent"         android:layout_height="wrap_content"         android:padding="5dp">          <TableRow>                        <EditText                 android:id="@+id/a_00"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_01"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_02"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_03"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_04"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_05"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_06"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_07"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_08"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />          </TableRow>          <TableRow >              <EditText                 android:id="@+id/a_10"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_11"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_12"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_13"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_14"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_15"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:inputType="number"                 android:gravity="center"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_16"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_17"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_18"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />          </TableRow>          <TableRow>              <EditText                 android:id="@+id/a_20"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_21"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_22"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_23"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_24"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_25"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_26"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_27"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_28"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />          </TableRow>          <TableRow             android:layout_marginTop="16dp">              <EditText                 android:id="@+id/a_30"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_31"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_32"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_33"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_34"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_35"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_36"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_37"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_38"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />         </TableRow>          <TableRow>              <EditText                 android:id="@+id/a_40"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_41"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_42"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_43"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_44"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_45"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_46"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_47"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_48"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />          </TableRow>          <TableRow>              <EditText                 android:id="@+id/a_50"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_51"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_52"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_53"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_54"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_55"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_56"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_57"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_58"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />          </TableRow>          <TableRow android:layout_marginTop="16dp">              <EditText                 android:id="@+id/a_60"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_61"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_62"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_63"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_64"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_65"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_66"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_67"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_68"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />          </TableRow>          <TableRow>              <EditText                 android:id="@+id/a_70"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_71"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_72"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_73"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_74"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_75"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_76"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_77"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_78"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />          </TableRow>          <TableRow>              <EditText                 android:id="@+id/a_80"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_81"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_82"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>              <EditText                 android:id="@+id/a_83"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_84"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_85"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number"                 android:layout_marginRight="22dp"/>                          <EditText                 android:id="@+id/a_86"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_87"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />              <EditText                 android:id="@+id/a_88"                 android:layout_width="0dp"                 android:layout_height="45dp"                 android:layout_weight="1"                 android:gravity="center"                 android:inputType="number" />          </TableRow>            </TableLayout>      <LinearLayout         android:layout_width="match_parent"         android:layout_height="wrap_content"         android:orientation="horizontal"         android:layout_marginTop="32dp">                <Button             android:id="@+id/solve"             android:layout_width="0dp"             android:layout_weight="1"             android:layout_height="wrap_content"             android:background="#283593"             android:padding="5dp"             android:layout_margin="8dp"             android:text="SOLVE"             android:textStyle="bold"             android:textSize="20sp"/>                <Button             android:id="@+id/clear"             android:layout_width="0dp"             android:layout_weight="1"             android:layout_height="wrap_content"             android:background="#283593"             android:padding="5dp"             android:layout_margin="8dp"             android:text="CLEAR"             android:textStyle="bold"             android:textSize="20sp"/>            </LinearLayout>    </LinearLayout>

Step 3: Working with the MainActivity.kt file

Go to the MainActivity.kt file and refer to the following code. Below is the code for the MainActivity.kt file. Comments are added inside the code to understand the code in more detail.

Inside MainActivity class declare the binding variable so that it can be used globally

Kotlin 
private lateinit var binding: ActivityMainBinding

Inside the onCreate method, we will use a 2d array of string to store the input and a 1d array that contains all the ids of editTexts.

Kotlin 
super.onCreate(savedInstanceState)         binding = ActivityMainBinding.inflate(layoutInflater)         setContentView(binding.root)          val matrix = arrayOf(             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),         )          var arr = ArrayList<EditText>()         binding.apply {             arr.addAll(                 arrayOf(                     a00, a01, a02, a03, a04, a05, a06, a07, a08,                     a10, a11, a12, a13, a14, a15, a16, a17, a18,                     a20, a21, a22, a23, a24, a25, a26, a27, a28,                     a30, a31, a32, a33, a34, a35, a36, a37, a38,                     a40, a41, a42, a43, a44, a45, a46, a47, a48,                     a50, a51, a52, a53, a54, a55, a56, a57, a58,                     a60, a61, a62, a63, a64, a65, a66, a67, a68,                     a70, a71, a72, a73, a74, a75, a76, a77, a78,                     a80, a81, a82, a83, a84, a85, a86, a87, a88,                 )             )         }

Now when the user types in the input integers and click solve then we have to first check for the validity of sudoku, so continuing the above code in onCreate

Kotlin 
 binding.solve.setOnClickListener {             var k = 0             for (i in 0..8) {                 for (j in 0..8) {                     if (arr[k].text.isNotEmpty()) {                         matrix[i][j] = arr[k].text.toString()                     }                     k++                 }             }                          if (!isValidConfig(matrix)) {                 Clear(matrix, arr)                 Toast.makeText(this@MainActivity, "Invalid Sudoku", Toast.LENGTH_SHORT)                     .show()             } else {                 solve(matrix, 0, 0)                 k = 0                 for (i in 0..8) {                     for (j in 0..8) {                         arr[k].setText(matrix[i][j])                         k++                     }                 }             }           }         binding.clear.setOnClickListener {             Clear(matrix, arr)         }

Now write the below code outside onCreate for validity check

Kotlin 
fun isValidConfig(arr: Array<Array<String>>): Boolean {         val n=arr.size         for (i in 0..n - 1) {             for (j in 0..n - 1) {                 if (arr[i][j]!="" && !valid(arr, i, j)) return false             }         }         return true     } private fun valid(arr: Array<Array<String>>, row: Int, col: Int): Boolean {         return (arr[row][col] !in "1".."9") && notInRow(arr, row) && notInCol(arr, col) &&                 notInBox(arr, row - row % 3, col - col % 3) } fun notInRow(matrix: Array<Array<String>>, row: Int): Boolean {          val st = HashSet<String>()         for (i in 0..8) {              if (st.contains(matrix[row][i])) return false              if (matrix[row][i] != "") st.add(matrix[row][i])         }         return true }  fun notInCol(arr: Array<Array<String>>, col: Int): Boolean {         val st = HashSet<String>()         for (i in 0..8) {              if (st.contains(arr[i][col])) return false              if (arr[i][col] != "") st.add(arr[i][col])         }         return true }  fun notInBox(         arr: Array<Array<String>>,         startRow: Int,         startCol: Int     ): Boolean {         val st = HashSet<String>()         for (row in 0..2) {             for (col in 0..2) {                 val curr = arr[row + startRow][col + startCol]                  if (st.contains(curr)) return false                  if (curr != "") st.add(curr)             }         }         return true  }  // on clicking the clear button to clear sudoku  private fun Clear(matrix: Array<Array<String>>, arr: ArrayList<EditText>) {         var k = 0         for (i in 0..8) {             for (j in 0..8) {                 matrix[i][j] = ""                 arr[k].setText("")                 k++             }         }     }

If the sudoku is invalid toast will be printed else if it is valid below code will be executed

Kotlin 
private fun solve(matrix: Array<Array<String>>, i: Int, j: Int): Boolean {         if (i == 9) return true         if (j == 9) return solve(matrix, i + 1, 0)         if (matrix[i][j] != "") return solve(matrix, i, j + 1)            for (it in 1..9) {             if (isvalid(matrix, i, j, it)) {                 matrix[i][j] = "$it"                 if (solve(matrix, i, j + 1)) return true                 matrix[i][j] = ""             }          }         return false     }      private fun isvalid(matrix: Array<Array<String>>, i: Int, j: Int, value: Int): Boolean {         val row = i - (i % 3)         val col = j - (j % 3)          for (x in 0..8) if (matrix[x][j] == "$value") return false         for (y in 0..8) if (matrix[i][y] == "$value") return false         for (x in 0..2) for (y in 0..2) if (matrix[row + x][col + y] == "$value"         ) return false         return true      }

Complete MainActivity Code:

Kotlin 
package com.example.sudokusolver20  import android.os.Bundle import android.widget.EditText import android.widget.Toast import androidx.appcompat.app.AppCompatActivity import com.example.sudokusolver20.databinding.ActivityMainBinding  class MainActivity : AppCompatActivity() {      private lateinit var binding: ActivityMainBinding     override fun onCreate(savedInstanceState: Bundle?) {         super.onCreate(savedInstanceState)         binding = ActivityMainBinding.inflate(layoutInflater)         setContentView(binding.root)          val matrix = arrayOf(             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),             arrayOf("", "", "", "", "", "", "", "", ""),         )          var arr = ArrayList<EditText>()         binding.apply {             arr.addAll(                 arrayOf(                     a00, a01, a02, a03, a04, a05, a06, a07, a08,                     a10, a11, a12, a13, a14, a15, a16, a17, a18,                     a20, a21, a22, a23, a24, a25, a26, a27, a28,                     a30, a31, a32, a33, a34, a35, a36, a37, a38,                     a40, a41, a42, a43, a44, a45, a46, a47, a48,                     a50, a51, a52, a53, a54, a55, a56, a57, a58,                     a60, a61, a62, a63, a64, a65, a66, a67, a68,                     a70, a71, a72, a73, a74, a75, a76, a77, a78,                     a80, a81, a82, a83, a84, a85, a86, a87, a88,                 )             )         }         binding.solve.setOnClickListener {             var k = 0             for (i in 0..8) {                 for (j in 0..8) {                     if (arr[k].text.isNotEmpty()) {                         matrix[i][j] = arr[k].text.toString()                     }                     k++                 }             }             if (!isValidConfig(matrix)) {                 Clear(matrix, arr)                 Toast.makeText(this@MainActivity, "Invalid Sudoku", Toast.LENGTH_SHORT)                     .show()             } else {                 solve(matrix, 0, 0)                 k = 0                 for (i in 0..8) {                     for (j in 0..8) {                         arr[k].setText(matrix[i][j])                         k++                     }                 }             }           }         binding.clear.setOnClickListener {             Clear(matrix, arr)         }       }      private fun Clear(matrix: Array<Array<String>>, arr: ArrayList<EditText>) {         var k = 0         for (i in 0..8) {             for (j in 0..8) {                 matrix[i][j] = ""                 arr[k].setText("")                 k++             }         }     }      private fun solve(matrix: Array<Array<String>>, i: Int, j: Int): Boolean {         if (i == 9) return true         if (j == 9) return solve(matrix, i + 1, 0)         if (matrix[i][j] != "") return solve(matrix, i, j + 1)            for (it in 1..9) {             if (isvalid(matrix, i, j, it)) {                 matrix[i][j] = "$it"                 if (solve(matrix, i, j + 1)) return true                 matrix[i][j] = ""             }          }         return false     }      private fun isvalid(matrix: Array<Array<String>>, i: Int, j: Int, value: Int): Boolean {         val row = i - (i % 3)         val col = j - (j % 3)          for (x in 0..8) if (matrix[x][j] == "$value") return false         for (y in 0..8) if (matrix[i][y] == "$value") return false         for (x in 0..2) for (y in 0..2) if (matrix[row + x][col + y] == "$value"         ) return false         return true      }       fun notInRow(matrix: Array<Array<String>>, row: Int): Boolean {          val st = HashSet<String>()         for (i in 0..8) {              if (st.contains(matrix[row][i])) return false              if (matrix[row][i] != "") st.add(matrix[row][i])         }         return true     }       fun notInCol(arr: Array<Array<String>>, col: Int): Boolean {         val st = HashSet<String>()         for (i in 0..8) {              if (st.contains(arr[i][col])) return false              if (arr[i][col] != "") st.add(arr[i][col])         }         return true     }      fun notInBox(         arr: Array<Array<String>>,         startRow: Int,         startCol: Int     ): Boolean {         val st = HashSet<String>()         for (row in 0..2) {             for (col in 0..2) {                 val curr = arr[row + startRow][col + startCol]                  if (st.contains(curr)) return false                  if (curr != "") st.add(curr)             }         }         return true     }      fun isValidConfig(arr: Array<Array<String>>): Boolean {         val n=arr.size         for (i in 0..n - 1) {             for (j in 0..n - 1) {                 if (arr[i][j]!="" && !valid(arr, i, j)) return false             }         }         return true     }      private fun valid(arr: Array<Array<String>>, row: Int, col: Int): Boolean {         return (arr[row][col] !in "1".."9") && notInRow(arr, row) && notInCol(arr, col) &&                 notInBox(arr, row - row % 3, col - col % 3)     }  }

Output:




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