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How to reverse a string in place in c using pointers?

The interview question was to write a function called revstr which can take a string and reverse it without using a buffer string i.e involving pointers. How do I do this?

Answer 1

Iterate from beginning and end simultaneously, swap characters.

void revstr(char * str) {
  int right = strlen(str) - 1;
  int left = 0;
  while (left < right) {
    char c = str[right];
    str[right] = str[left];
    str[left] = c;
    ++left;
    --right;
  }
}

Optionally you can use xor tricks to swap without an intermediate char:

str[right] ^= str[left];
str[left] ^= str[right];
str[right] ^= str[left];

This is a purely nonsensical way of doing a swap – the only reason to use this construct is an artificial requirement saying that you can’t store string data in intermediate variables and you can’t call external functions.