Reverse digits of an integer with overflow handled

Last Updated : 23 Apr, 2025

Write a program to reverse an integer assuming that the input is a 32-bit integer. If the reversed integer overflows, print -1 as the output.

Let us see a simple approach to reverse digits of an integer.

C++

// A simple C program to reverse digits of // an integer. #include <bits/stdc++.h> using namespace std;  int reversDigits(int num) {     int rev_num = 0;     while (num != 0) {         rev_num = rev_num * 10 + num % 10;         num = num / 10;     }     return rev_num; }  /* Driver program to test reversDigits */ int main() {     int num = 5896;     cout << "Reverse of no. is " << reversDigits(num);     return 0; }  // This code is contributed by Sania Kumari Gupta (kriSania804) // This code is improved by Md. Owais Ashraf (professor_011)

// A simple C program to reverse digits of // an integer. #include <stdio.h>  int reversDigits(int num) {     int rev_num = 0;     while (num != 0) {         rev_num = rev_num * 10 + num % 10;         num = num / 10;     }     return rev_num; }  /* Driver program to test reversDigits */ int main() {     int num = 5896;     printf("Reverse of no. is %d", reversDigits(num));     return 0; }  // This code is contributed by Sania Kumari Gupta (kriSania804) // This code is improved by Md. Owais Ashraf (professor_011)

Java

// Java program to reverse a number   class GFG {     /* Iterative function to reverse     digits of num*/     static int reversDigits(int num)     {         int rev_num = 0;         while(num!=0)         {             rev_num = rev_num * 10 + num % 10;             num = num / 10;         }         return rev_num;     }          // Driver code     public static void main (String[] args)      {         int num = 5896;         System.out.println("Reverse of no. is "                             + reversDigits(num));     } }  // This code is contributed by Anant Agarwal. // This code is improved by Md. Owais Ashraf (professor_011) // This code is improved by Jeet Purohit  (jeetpurohit989)

Python

# Python program to reverse a number   n = 5896; rev = 0  while(n != 0):     a = n % 10     rev = rev * 10 + a     n = n // 10      print(rev)  # This code is contributed by Shariq Raza  # This code is improved by Jeet Purohit  (jeetpurohit989)

// C# program to reverse a number  using System;  class GFG {     // Iterative function to      // reverse digits of num     static int reversDigits(int num)     {         int rev_num = 0;         while(num != 0)         {             rev_num = rev_num * 10 + num % 10;             num = num / 10;         }         return rev_num;     }          // Driver code     public static void Main()      {         int num = 5896;         Console.Write("Reverse of no. is "                         + reversDigits(num));     } }  // This code is contributed by Sam007 // This code is improved by Md. Owais Ashraf (professor_011)

JavaScript

<script>  // A simple Javascript program to reverse digits of  // an integer.   function reversDigits(num)  {      let rev_num = 0;      while (num != 0)      {          rev_num = rev_num*10 + num%10;          num = Math.floor(num/10);      }      return rev_num;  }   /* Driver program to test reversDigits */       let num = 5896;      document.write("Reverse of no. is "+ reversDigits(num));       // This code is contributed by Mayank Tyagi // This code is improved by Md. Owais Ashraf (professor_011) // This code is improved by Jeet Purohit  (jeetpurohit989) </script>

PHP

<?php // Iterative function to  // reverse digits of num function reversDigits($num) {     $rev_num = 0;     while($num != 0)     {         $rev_num = $rev_num * 10 +                          $num % 10;         $num = (int)$num / 10;     }     return $rev_num/10; }  // Driver Code $num = 5896; echo "Reverse of no. is ",         reversDigits($num);  // This code is contributed by aj_36 // This code is improved by Md. Owais Ashraf (professor_011) // This code is improved by Jeet Purohit  (jeetpurohit989)  ?>

Output

Reverse of no. is 6985

Time Complexity: O(log(num))
Auxiliary Space: O(1)

Optimized Approach:

However, if the number is large such that the reverse overflows, the output is some garbage value. If we run the code above with input as any large number say 1000000045, then the output is some garbage value like 1105032705 or any other garbage value.
How to handle overflow?
The idea is to store the previous value of the sum can be stored in a variable that can be checked every time to see if the reverse overflowed or not.

Below is the implementation to deal with such a situation.

C++

// C++ program to reverse digits  // of a number #include <bits/stdc++.h>  using namespace std;  /* Iterative function to reverse  digits of num*/ int reversDigits(int num) {     // Handling negative numbers     bool negativeFlag = false;     if (num < 0)     {         negativeFlag = true;         num = -num ;     }      int prev_rev_num = 0, rev_num = 0;     while (num != 0)     {         int curr_digit = num % 10;          rev_num = (rev_num * 10) + curr_digit;          // checking if the reverse overflowed or not.         // The values of (rev_num - curr_digit)/10 and         // prev_rev_num must be same if there was no         // problem.         if ((rev_num - curr_digit) /                 10 != prev_rev_num)         {             cout << "WARNING OVERFLOWED!!!"                  << endl;             return 0;         }          prev_rev_num = rev_num;         num = num / 10;     }      return (negativeFlag == true) ?                          -rev_num : rev_num; }  // Driver Code int main() {     int num = 12345;     cout << "Reverse of no. is "           << reversDigits(num) << endl;      num = 1000000045;     cout << "Reverse of no. is "           << reversDigits(num) << endl;      return 0; }  // This code is contributed  // by Akanksha Rai(Abby_akku)

// C program to reverse digits of a number #include <stdio.h>  /* Iterative function to reverse digits of num*/ int reversDigits(int num) {     // Handling negative numbers     bool negativeFlag = false;     if (num < 0)     {         negativeFlag = true;         num = -num ;     }      int prev_rev_num = 0, rev_num = 0;     while (num != 0)     {         int curr_digit = num%10;          rev_num = (rev_num*10) + curr_digit;          // checking if the reverse overflowed or not.         // The values of (rev_num - curr_digit)/10 and         // prev_rev_num must be same if there was no         // problem.         if ((rev_num - curr_digit)/10 != prev_rev_num)         {             printf("WARNING OVERFLOWED!!!\n");             return 0;         }          prev_rev_num = rev_num;         num = num/10;     }      return (negativeFlag == true)? -rev_num : rev_num; }  /* Driver program to test reverse Digits */ int main() {     int num = 12345;     printf("Reverse of no. is %d\n", reversDigits(num));      num = 1000000045;     printf("Reverse of no. is %d\n", reversDigits(num));      return 0; }

Java

// Java program to reverse digits of a number  class ReverseDigits {     /* Iterative function to reverse digits of num*/     static int reversDigits(int num)     {         // Handling negative numbers         boolean negativeFlag = false;         if (num < 0)         {             negativeFlag = true;             num = -num ;         }               int prev_rev_num = 0, rev_num = 0;         while (num != 0)         {             int curr_digit = num%10;                   rev_num = (rev_num*10) + curr_digit;                   // checking if the reverse overflowed or not.             // The values of (rev_num - curr_digit)/10 and             // prev_rev_num must be same if there was no             // problem.             if ((rev_num - curr_digit)/10 != prev_rev_num)             {                 System.out.println("WARNING OVERFLOWED!!!");                 return 0;             }                   prev_rev_num = rev_num;             num = num/10;         }               return (negativeFlag == true)? -rev_num : rev_num;     }          public static void main (String[] args)      {         int num = 12345;         System.out.println("Reverse of no. is " + reversDigits(num));               num = 1000000045;         System.out.println("Reverse of no. is " + reversDigits(num));     } }

Python

# Python program to reverse digits  # of a number   """ Iterative function to reverse  digits of num""" def reversDigits(num):       # Handling negative numbers      negativeFlag = False     if (num < 0):              negativeFlag = True         num = -num            prev_rev_num = 0     rev_num = 0     while (num != 0):               curr_digit = num % 10          rev_num = (rev_num * 10) + curr_digit           # checking if the reverse overflowed or not.          # The values of (rev_num - curr_digit)/10 and          # prev_rev_num must be same if there was no          # problem.         if (rev_num >= 2147483647 or              rev_num <= -2147483648):             rev_num = 0         if ((rev_num - curr_digit) // 10 != prev_rev_num):                      print("WARNING OVERFLOWED!!!")              return 0                   prev_rev_num = rev_num          num = num //10           return -rev_num if (negativeFlag == True) else rev_num      # Driver code  if __name__ =="__main__":     num = 12345     print("Reverse of no. is ",reversDigits(num))       num = 1000000045     print("Reverse of no. is ",reversDigits(num))                # This code is contributed # Shubham Singh(SHUBHAMSINGH10)

// C# program to reverse digits // of a number using System;  class GFG {  /* Iterative function to reverse    digits of num*/ static int reversDigits(int num)  {      // Handling negative numbers      bool negativeFlag = false;      if (num < 0)      {          negativeFlag = true;          num = -num ;      }       int prev_rev_num = 0, rev_num = 0;      while (num != 0)      {          int curr_digit = num % 10;           rev_num = (rev_num * 10) +                     curr_digit;           // checking if the reverse overflowed          // or not. The values of (rev_num -          // curr_digit)/10 and prev_rev_num must          // be same if there was no problem.          if ((rev_num - curr_digit) / 10 != prev_rev_num)          {              Console.WriteLine("WARNING OVERFLOWED!!!");              return 0;          }           prev_rev_num = rev_num;          num = num / 10;      }       return (negativeFlag == true) ?                          -rev_num : rev_num;  }   // Driver Code static public void Main () {     int num = 12345;      Console.WriteLine("Reverse of no. is " +                           reversDigits(num));       num = 1000000045;      Console.WriteLine("Reverse of no. is " +                           reversDigits(num));  } }  // This code is contributed by ajit

JavaScript

<script> // JavaScript program to reverse digits // of a number  /* Iterative function to reverse digits of num*/ function reversDigits(num) {     // Handling negative numbers     let negativeFlag = false;     if (num < 0)     {         negativeFlag = true;         num = -num ;     }      let prev_rev_num = 0, rev_num = 0;     while (num != 0)     {         let curr_digit = num % 10;          rev_num = (rev_num * 10) + curr_digit;          // checking if the reverse overflowed or not.         // The values of (rev_num - curr_digit)/10 and         // prev_rev_num must be same if there was no         // problem.         if (rev_num >= 2147483647 ||             rev_num <= -2147483648)             rev_num = 0;         if (Math.floor((rev_num - curr_digit) / 10) != prev_rev_num)         {             document.write("WARNING OVERFLOWED!!!"                 + "<br>");             return 0;         }          prev_rev_num = rev_num;         num = Math.floor(num / 10);     }      return (negativeFlag == true) ?                         -rev_num : rev_num; }  // Driver Code     let num = 12345;     document.write("Reverse of no. is "         + reversDigits(num) + "<br>");      num = 1000000045;     document.write("Reverse of no. is "         + reversDigits(num) + "<br>");      // This code is contributed by Surbhi Tyagi. </script>

Output

Reverse of no. is 54321 WARNING OVERFLOWED!!! Reverse of no. is 0

Time Complexity: O(log(num))
Auxiliary Space: O(1)

Efficient Approach:

The above approach won’t work if we are given a signed 32-bit integer x, and return x with its digits reversed. If reversing x causes the value to go outside the signed 32-bit integer range [-231, 231 – 1], then return 0. So we cannot multiply the number*10 and then check if the number overflows or not.

We must check the overflow condition before multiplying by 10 by using the following logic :
You are checking the boundary case before you do the operation. (reversed >INT_MAX ) wouldn’t work because reversed will overflow and become negative if it goes past MAX_VALUE. Dividing MAX_VALUE by 10 lets you check the condition without overflowing INT_MAX is equal 2147483647. INT_MIN is equal -2147483648. The last digits are 7 and 8. This is the reason why we also check rem > 7 and rem < -8 conditions

Implementation:

C++

// C++ program to reverse digits  // of a number #include <bits/stdc++.h>  using namespace std; int reversDigits(int num) {          int rev = 0  ;          while(num != 0){                  int rem = num % 10 ;         num /= 10 ;                  if(rev > INT_MAX/10 || rev < INT_MIN/10){             return 0 ;         }                  rev = rev*10 + rem ;     }          return rev ;      }  // Driver Code int main() {     int num = 12345;     cout << "Reverse of no. is "           << reversDigits(num) << endl;      num = 1000000045;     cout << "Reverse of no. is "           << reversDigits(num) << endl;      return 0; }

Java

// Java program to reverse digits // of a number public class GFG {    static int reversDigits(int num)   {     int rev = 0  ;      while(num != 0){               int rem = num % 10 ;       num /= 10 ;        if(rev > Integer.MAX_VALUE/10 || rev == Integer.MAX_VALUE/10 && rem > 7){         return 0 ;       }        if(rev < Integer.MIN_VALUE/10 || rev == Integer.MIN_VALUE/10 && rem < -8){         return 0 ;       }        rev = rev*10 + rem ;     }      return rev ;   }    // Driver code   public static void main (String[] args)   {     int num = 12345;     System.out.println("Reverse of no. is " + reversDigits(num) );      num = 1000000045;     System.out.println("Reverse of no. is " + reversDigits(num) );   } }  // This code is contributed by jana_sayantan.

Python

def reverse_digits(num):     # Initialize reverse number as 0     rev = 0      # Check sign of the number     sign = -1 if num < 0 else 1      # Take absolute value of the number     num = abs(num)      # Reverse the digits     while num != 0:         # Get the last digit         rem = num % 10          # Remove the last digit from the number         num = num // 10          # Check if the reverse number will overflow 32-bit integer         if rev > 2**31//10 or (rev == 2**31//10 and rem > 7):             return 0          # Check if the reverse number will underflow 32-bit integer         if rev < -2**31//10 or (rev == -2**31//10 and rem < -8):             return 0          # Update the reverse number         rev = rev * 10 + rem      # Return the reverse number with the original sign     return sign * rev  if __name__ == '__main__':     num = 12345     print("Reverse of no. is", reverse_digits(num))      num = 1000000045     print("Reverse of no. is", reverse_digits(num))

// C# program to reverse digits // of a number using System;  public class GFG {    static int reversDigits(int num)   {     int rev = 0  ;      while(num != 0){               int rem = num % 10 ;       num /= 10 ;        if(rev > Int32.MaxValue/10 || rev == Int32.MaxValue/10 && rem > 7){         return 0 ;       }        if(rev < Int32.MinValue/10 || rev == Int32.MinValue/10 && rem < -8){         return 0 ;       }        rev = rev*10 + rem ;     }      return rev ;   }    // Driver code   public static void Main (string[] args)   {     int num = 12345;     Console.WriteLine("Reverse of no. is " + reversDigits(num) );      num = 1000000045;     Console.WriteLine("Reverse of no. is " + reversDigits(num) );   } }  // This code is contributed by phasing17

JavaScript

<script>  // JavaScript program to reverse digits  // of a number  const INT_MAX = 2147483647; const INT_MIN = -2147483648;  function reversDigits(num) {          let rev = 0;          while(num > 0){                  let rem = num % 10 ;         num = Math.floor(num/10) ;                  if(rev > INT_MAX/10 || rev == INT_MAX/10 && rem > 7){             return 0 ;         }                  if(rev < INT_MIN/10 || rev == INT_MIN/10 && rem < -8){             return 0 ;         }                  rev = rev*10 + rem ;      }          return rev ;      }  // Driver Code  let num = 12345; document.write(`Reverse of no. is ${reversDigits(num)}`,"</br>");  num = 1000000045; document.write(`Reverse of no. is ${reversDigits(num)}`,"</br>");  // This code is contributed by shinjanpatra  </script>

Output

Reverse of no. is 54321 Reverse of no. is 0

Time Complexity: O(log(num))
Auxiliary Space: O(1)

Approach using slice method:

We convert the integer to a string, reverse the string using the slice operator, and then convert the reversed string back to an integer.

Steps:

Convert the integer to a string using the str() function
Use the slice operator [::-1] to reverse the string
Convert the reversed string back to an integer using the int() function

Implementation:

C++

#include <iostream> #include <string>  int reverseNumStringSlice(int num) {     // Convert the integer to a string     std::string numStr = std::to_string(num);      // Reverse the string using the slice operator     std::string reversedStr(numStr.rbegin(), numStr.rend());      // Convert the reversed string back to an integer     int reversedNum = std::stoi(reversedStr);      return reversedNum; }  int main() {     int num = 12345;     int revNumStringSlice = reverseNumStringSlice(num);     std::cout << "Reverse of " << num << " is "               << revNumStringSlice << std::endl;      return 0; }

Java

public class ReverseNumStringSlice {     // Function to reverse an integer using string slice     static int reverseNumStringSlice(int num)     {         // Convert the integer to a string         String numStr = Integer.toString(num);          // Reverse the string using the substring method         String reversedStr = new StringBuilder(numStr)                                  .reverse()                                  .toString();          // Convert the reversed string back to an integer         int reversedNum = Integer.parseInt(reversedStr);          return reversedNum;     }      // Main method     public static void main(String[] args)     {         // Example integer         int num = 12345;          // Call the reverseNumStringSlice function         int revNumStringSlice = reverseNumStringSlice(num);          // Output the result         System.out.println("Reverse of " + num + " is "                            + revNumStringSlice);     } }

Python

def reverse_num_string_slice(num):     return int(str(num)[::-1])  num = 12345 rev_num_string_slice = reverse_num_string_slice(num) print(f"Reverse of {num} is {rev_num_string_slice}")

using System;  class Program {     static int ReverseNumStringSlice(int num)     {         // Convert the integer to a string         string numStr = num.ToString();          // Reverse the string using the slice operator         char[] charArray = numStr.ToCharArray();         Array.Reverse(charArray);         string reversedStr = new string(charArray);          // Convert the reversed string back to an integer         int reversedNum = int.Parse(reversedStr);          return reversedNum;     }      static void Main()     {         int num = 12345;         int revNumStringSlice = ReverseNumStringSlice(num);         Console.WriteLine("Reverse of " + num + " is "                           + revNumStringSlice);     } }

JavaScript

function GFG(num) {     // Convert the integer to a string     const numStr = num.toString();     // Reverse the string using slice operator     const reversedStr = numStr.split('').reverse().join('');     // Convert the reversed string back to an integer     const reversedNum = parseInt(reversedStr, 10);     return reversedNum; } // Main function function main() {     const num = 12345;     // Call the function to the reverse the number     const revNumStringSlice = GFG(num);     // Display the result     console.log(`Reverse of ${num} is ${revNumStringSlice}`); } main();

Output

Reverse of 12345 is 54321

Time complexity: O(n), where n is the number of digits in the integer.
Space complexity: O(n), where n is the number of digits in the integer, since we need to create a string of length n to store the reversed integer.

Number of digits in 2 raised to power n

MAZHAR IMAM KHAN

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Reverse digits of an integer with overflow handled

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