How to swap two integer

Last week, I was on a job interview and given a challenge, how to swap an integer value between two variables. So given this code:

var a = 10
var b = 2

I should think how to swap a and b values. The first solution that comes up in my mind, some like this:

a = [a, b]
a = a[0]
b = a[1]

Turn out that I can't change the datatype, LOL. So, after solid 3 minutes, I propose another solution:

a = 2
b = 10

LMFAO. Basically it works fine and correct but can't be used if a and/or b have different value. After 10 minutes without a solution, I gave up and they show me the correct answer. It's so fucking simple.

a = a + b   // 12
b = a - b   // 10
a = a - b   // 2