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Reverse digits of an integer with overflow handled

  • Difficulty Level : Medium
  • Last Updated : 01 Dec, 2021

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
 

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C++




// 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;
}


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 = 4562;
        System.out.println("Reverse of no. is "
                           + reversDigits(num));
    }
}
 
// This code is contributed by Anant Agarwal.


Python




# Python program to reverse a number
 
n = 4562;
rev = 0
 
while(n > 0):
    a = n % 10
    rev = rev * 10 + a
    n = n / 10
     
print(rev)
 
# This code is contributed by Shariq Raza


C#




// 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 = 4562;
        Console.Write("Reverse of no. is "
                        + reversDigits(num));
    }
}
 
// This code is contributed by Sam007


PHP




<?php
// Iterative function to
// reverse digits of num
function reversDigits($num)
{
    $rev_num = 0;
    while($num > 1)
    {
        $rev_num = $rev_num * 10 +
                        $num % 10;
        $num = (int)$num / 10;
    }
    return $rev_num;
}
 
// Driver Code
$num = 4562;
echo "Reverse of no. is ",
       reversDigits($num);
 
// This code is contributed by aj_36
?>


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
 
</script>


Output

Reverse of no. is 6985

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. See this for the output.
How to handle overflow? 
The idea is to store previous value of the sum can be stored in a variable which 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




// 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));
    }
}


Python3




# 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#




// 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

Efficient Approach :

The above approach won’t work if we are given a signed 32-bit integer x, 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 multiply 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. 

Last digits are 7 and 8. This is the reason why we also  check  rem > 7 and rem < -8 conditions

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_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
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;
}


Output

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

This article is contributed by MAZHAR IMAM KHAN. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
 




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