70. Climbing Stairs

Input: 2
Output:  2
Explanation:  There are two ways to climb to the top.

1. 1 step + 1 step
2. 2 steps

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

动态规划：dp
=dp[n-1]+dp[n-2]，弄得此题类似斐波那契数列。。。

class Solution:
def climbStairs(self, n):
"""
:type n: int
:rtype: int
"""
a,b=0,1
for i in range(n):
a,b=b,a+b
return b