Reverse a string in C using as little additional memory as possible

Problem

Question: Reverse a string in C using as little additional memory as possible.

Solution

Answer: The first solution needs the size of a char and size of two integers, all of which will be allocated from the stack. This solution is the most commonly accepted “good” solution. Here is the code.

Method 1 - Using 2 pointer technique

We have already seen Reverse String Problem#Method 1 - Iterative in-place using Two Pointers

Method 2 - Using xor to swap 2 characters

The second solution is slightly better than the first as it does not need the char space. It uses bitmanipulation (XOR in this case) to swap the chars in place.

void reverseStringBetter(char* str)
{
    int i, j;
    i=j=0;

    j=strlen(str)-1;
    for (i=0; i<j; i++, j--)
    {
        str[i] ^= str[j] ;
        str[j] ^= str[i] ;
        str[i] ^= str[j] ;
    }
}

Basically it is using Swap two number in place without temporary variables. Even though the second solution uses less space, the first one is probably a better solution in terms of readability and maintainability. The compilers are very advanced now and are able to reduce the swap instructions in the first solution into a single execution step. However, the same might not happen for the second solution so in real life situations, it is better to use the first solution although the second solution seems smarter and better theoretically.