Given a string, reverse it without using any temporary variables.

This problem is a variation of the problem swapping two integers without using any temporary variables by XOR.

For integers x and y, to swap them using XOR, we do the following.

x = x ^ y;

y = y ^ x;

x = x ^ y;

Proof of correctness.

y = y ^ (x ^ y) = (y ^ y) ^ x = 0 ^ x = x;

x = (x ^ y) ^ x = (x ^ x) ^ y = 0 ^ y = y;

QED.

Apply this 3 steps XOR operations to each pair of characters that need to be swapped.

 1 public class Solution {
 2     public char[] reverseString(char[] str, int start, int end){
 3         while(start < end){
 4             str[start] ^= str[end];
 5             str[end] ^= str[start];
 6             str[start] ^= str[end];
 7             start++;
 8             end--;
 9         }
10         return str;
11     }
12 }

[GeeksForGeeks] Reverse a string without using any temporary variables