70. Climbing Stairs

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?

Note: Given n will be a positive integer.

 

class Solution(object):
    def climbStairs(self, n):
        """
        :type n: int
        :rtype: int
        """
        a = b = 1
        for _ in range(n):
            a, b = b, a + b
        return a

a：the number of ways to reach the current step

b: the number of ways to reach the next step