Swap two numbers. Thought process behind this: You aren't able to store data in any new variables, so the only option available to you is the stack.

swap.c

// Assume a < b
void swap(int &a, int &b)
{
  if(a < b) {
    a++;
    swap(a, b);
    b--;
  }
}

// If you do not want to assume a < b, you can add the following snippet to the top of the function
// if(b < a) return swap(b, a);