How many different ways can you reach the nth stair if you can take either one or two steps at a time?
Implement climb_stairs(n) to return the number of ordered step sequences that reach stair n, with climb_stairs(0) = 1. For example, n = 2 returns 2 and n = 5 returns 8; assume 0 <= n <= 50 and return an integer.
def climb_stairs(n):