The classic implementation of the swap algorithm looks like this:

   void swap (int & i, int & j)  {    int temp = i;    i = j;    j = temp;  }

Is it possible to swap i and j without using a third variable? Yes it is:

   swap(int & i, int & j)  {    i -= j;    j += i; // j gets the original value of i    i = (j - i); // i gets the original value of j  }

However, this version isn’t more efficient than the previous one (in fact, it’s probably less efficient) because the compiler must generate a temporary variable to evaluate the following expression:

   i = (j - i); 

Although it has no practical value, this exercise may show up in your next job interview so it’s worth remembering how to solve it.