Leetcode – 7 – Reverse Integer

Reverse Integer

Reverse digits of an integer.

Example1: x = 123, return 321
Example2: x = -123, return -321


In this problem we have to reverse the integer given. for negative number the integer should be reversed with a sign.

Efficient solution: (Using Modulo and repeated division)

When we take modulo (%) of 10 for the number we will get the last digit. for ex

  • 321 % 10 will give us 1
  • 32 % 10 will give us 2
  • 3 % 10 will give us 3

So in this case we have to do repeated modulo and store it in a variable

Also after taking modulo of the first number we will again have to get the remaining digits

321 after modulo 1 we have to get 32 we can employ division (/) for this case. Repeated division will give us the reminders

So we derive at the following logic with this

  • loop until n == 0
  • keep the running variable to save the intermediate values of the modulo
  • every time multiply by 10 to get the correct digit places
  • each iteration divide the number



public class Solution {
    public int reverse(int x) {
         long result = 0;
         int n = x;
         for(;n != 0;n /= 10){
             result = result *10 + (n%10);
         return result > Integer.MAX_VALUE || result < Integer.MIN_VALUE ? 0: (int) result;

Time Complexity: O(n)

  • Loop through the digits upto n – O(n)

Space Complexity: O(1)

  • We will store the values in the result – O(1)

Available Test Cases: 1032

  • Check for no input
  • Check for overflow / underflow (use higher data type – long to define the results)
  • Check for duplicate digits
  • Check for negative number
  • Check for maximum and minimum




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