# Tail Recursion

In this blog post, you’ll learn to -

1. Understand the different types of recursion.
2. Understand why tail recursion is more effecient.

Consider the following recursive function to compute the GCD.

Here’s how the stack trace of this would look like -

gcd(14, 21) -> gcd (21, 14 % 21) -> gcd (14, 7) -> gcd (7, 0) -> 7

Let us look at another type of recursion - factorial.

factorial(4) -> 4 * factorial(3) -> 4 * 3 * factorial(2) -> 4 * 3 * 2 * factorial(1) -> 4 * 3 * 2 * 1 * factorial(0) -> 24

What’s the difference between the above 2 types of recursion. The expression in the factorial function grows as we go deeper in the recursion. However, for GCD the expression stays the same - it osciallates between one call to another.

If the function calls itself as its last action, the function’s stack frame could be reused. This is called tail recursion.

In tail recursion, the stack frame of the function could be reused and hence it could execute in constant stack space. JVM restrict the size of the stack frame to a couple of thousand and hence if you think that your function could have a lot of depth, it might be wise to re-implement it as tail-recursive.