0070-Climbing-Stairs.py

'''
You are climbing a stair case. It takes n steps to reach to the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

Example 1:

Input: 2
Output: 2
Explanation: There are two ways to climb to the top.
1. 1 step + 1 step
2. 2 steps
Example 2:

Input: 3
Output: 3
Explanation: There are three ways to climb to the top.
1. 1 step + 1 step + 1 step
2. 1 step + 2 steps
3. 2 steps + 1 step
 

Constraints:

1 <= n <= 45
'''
class Solution:
    def climbStairs(self, n):
        if n  == 1:
            return 1
        a, b = 1, 2
        for _ in range(2, n):
            a, b = b, a + b
        return b