k-th missing element in an unsorted array

Last Updated : 11 Aug, 2022

Given an unsorted sequence a[], the task is to find the K-th missing contiguous element in the increasing sequence of the array elements i.e. consider the array in sorted order and find the kth missing number. If no k-th missing element is there output -1.
Note: Only elements exists in the range of minimum and maximum element to be considered.
Examples:

Input: arr[] = {2, 4, 10, 7}, k = 5 Output: 9 Missing elements in the given array: 3, 5, 6, 8, 9 5th missing is 9.  Input: arr[] = {1, 3, 4}, k = 5 Output: -1

Method-1: Sort the array and use the approach used in the k-th missing element in a sorted array.
Method-2:

Insert all the elements in an unordered_set.
Find the minimum and maximum element of the array.
Traverse the elements from minimum to maximum.
- Check if current element is present in the set or not.
- If not then check if this is kth missing by counting the missing elements.
- Return the current element if this is current missing.

Below is the implementation of the above approach:

C++

// C++ implementation of the above approach #include <bits/stdc++.h> using namespace std;  // Function to find the sum // of minimum of all subarrays int findKth(int arr[], int n, int k) {      unordered_set<int> missing;     int count = 0;      // Insert all the elements in a set     for (int i = 0; i < n; i++)         missing.insert(arr[i]);      // Find the maximum and minimum element     int maxm = *max_element(arr, arr + n);     int minm = *min_element(arr, arr + n);      // Traverse from the minimum to maximum element     for (int i = minm + 1; i < maxm; i++) {         // Check if "i" is missing         if (missing.find(i) == missing.end())             count++;          // Check if it is kth missing         if (count == k)             return i;     }      // If no kth element is missing     return -1; }  // Driver code int main() {     int arr[] = { 2, 10, 9, 4 };     int n = sizeof(arr) / sizeof(arr[0]);     int k = 5;     cout << findKth(arr, n, k);      return 0; }

Java

// Java implementation of the above approach import java.util.*;  class GFG {      // Function to find the sum     // of minimum of all subarrays     static int findKth(int arr[], int n, int k)      {          HashSet<Integer> missing = new HashSet<>();         int count = 0;          // Insert all the elements in a set         for (int i = 0; i < n; i++)         {             missing.add(arr[i]);         }          // Find the maximum and minimum element         int maxm = Arrays.stream(arr).max().getAsInt();         int minm = Arrays.stream(arr).min().getAsInt();          // Traverse from the minimum to maximum element         for (int i = minm+1; i < maxm; i++)         {             // Check if "i" is missing             if (!missing.contains(i))              {                  count++;             }              // Check if it is kth missing             if (count == k)             {                  return i;             }         }                  // If no kth element is missing         return -1;     }      // Driver code     public static void main(String[] args)      {         int arr[] = {2, 10, 9, 4};         int n = arr.length;         int k = 5;         System.out.println(findKth(arr, n, k));     } }  /* This code contributed by PrinciRaj1992 */

Python

# Python3 implementation of the above approach  # Function to find the sum # of minimum of all subarrays def findKth( arr, n, k):      missing = dict()     count = 0      # Insert all the elements in a set     for i in range(n):         missing[arr[i]] = 1      # Find the maximum and minimum element     maxm = max(arr)     minm = min(arr)      # Traverse from the minimum to maximum element     for i in range(minm + 1, maxm):                  # Check if "i" is missing         if (i not in missing.keys()):             count += 1          # Check if it is kth missing         if (count == k):             return i          # If no kth element is missing     return -1  # Driver code arr = [2, 10, 9, 4 ] n = len(arr) k = 5 print(findKth(arr, n, k))  # This code is contributed by Mohit Kumar

// C# implementation of the above approach  using System; using System.Linq; using System.Collections.Generic;  class GFG  {       // Function to find the sum      // of minimum of all subarrays      static int findKth(int []arr, int n, int k)      {           HashSet<int> missing = new HashSet<int>();          int count = 0;           // Insert all the elements in a set          for (int i = 0; i < n; i++)          {              missing.Add(arr[i]);          }           // Find the maximum and minimum element          int maxm = arr.Max();          int minm = arr.Min();           // Traverse from the minimum to maximum element          for (int i = minm + 1; i < maxm; i++)          {              // Check if "i" is missing              if (!missing.Contains(i))              {                  count++;              }               // Check if it is kth missing              if (count == k)              {                  return i;              }          }                   // If no kth element is missing          return -1;      }       // Driver code      public static void Main(String[] args)      {          int []arr = {2, 10, 9, 4};          int n = arr.Length;          int k = 5;          Console.WriteLine(findKth(arr, n, k));      }  }   // This code has been contributed by 29AjayKumar

PHP

<?php // PHP implementation of the above approach   // Function to find the sum  // of minimum of all subarrays  function findKth($arr, $n, $k) {      $missing = array();     $count = 0;      // Insert all the elements in a set      for ($i = 0; $i < $n; $i++)         array_push($missing, $arr[$i]);      $missing = array_unique($missing);          // Find the maximum and minimum element      $maxm = max($arr);     $minm = min($arr);      // Traverse from the minimum to     // maximum element      for ($i = $minm + 1; $i < $maxm; $i++)     {         // Check if "i" is missing          if (!in_array($i, $missing, false))             $count += 1;          // Check if it is kth missing          if ($count == $k)              return $i;     }          // If no kth element is missing      return -1; }  // Driver code  $arr = array(2, 10, 9, 4);  $n = sizeof($arr);  $k = 5;  echo findKth($arr, $n, $k);  // This code is contributed by Ryuga ?>

JavaScript

<script>  // javascript implementation of the above approach  // Function to find the sum // of minimum of all subarrays function findKth(arr, n, k) {      var missing = new Set();     var count = 0;      // Insert all the elements in a set     for (var i = 0; i < n; i++)         missing.add(arr[i]);      // Find the maximum and minimum element     var maxm = arr.reduce((a,b)=>Math.max(a,b));     var minm = arr.reduce((a,b)=>Math.min(a,b));      // Traverse from the minimum to maximum element     for (var i = minm + 1; i < maxm; i++) {         // Check if "i" is missing         if (!missing.has(i))             count++;          // Check if it is kth missing         if (count == k)             return i;     }      // If no kth element is missing     return -1; }  // Driver code var arr = [ 2, 10, 9, 4 ]; var n = arr.length; var k = 5; document.write( findKth(arr, n, k));  // This code is contributed by noob2000. </script>

Output:

Time complexity: O(n) where n is size of input array

Auxiliary Space: O(n)

K’th Smallest Element in Unsorted Array

Shivam.Pradhan

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