Before half-century, each bit inside of the computer was important. One of the most frequently used function was "swap".
Swap do the simple thing and you can say well I know. But in that time each operation and memory usage was important. So to have an efficient swap between 2 variables and not using third variables.
So trick goes like this:
A = A + B B = A - B A = A - B
TMP = A A = B B = TMP
Python code (tnx to Jovan Sh):
>>> a = 5 >>> b = 3 >>> (a, b) = (b, a) >>> print a,b 3 5
With this approach at that time, programmers save a lot of memory usage.
Also, you can find all these tricks in the book Hacker's Delight