Swap values of variables without using intermediate variable

          Swap values of variables is basic practical question for programming beginners. It is very easy and usually we are doing it using a 3rd variable to store value temporary.

Algorithm by using 3rd variable:

tempVar = firstVar;
firstVar = secondVar;
secondVar = tempVar;

Now for a change one asks to you to do it by without using a 3rd variable. This question made a brain storm when I was studying basics of C programming. My wild thoughts didn't give the answer for first 1 minute. I would like to share the code.

#include <iostream>
using namespace std;
int main()
{
    int a, b;
    a = 10;     // variable `a` gets value of 10
    b = 20;     // variable `b` gets value of 20
    cout<<"Before swap, a : "<<a<<", b: "<<b<<endl;
    a = a+b;    // now `a` is 10 + 20 = 30
    b = a-b;    // now `b` is 30 - 20 = 10
    a = a-b;    // now `a` is 30 - 10 = 20 -----> Swapped values
    cout<<"After swap, a : "<<a<<", b : "<<b<<endl;
}