If you notice that **the corresponding results of each inputs from 0 to n can constitute the Fibonacci numbers**

i.e. 1,1,2,3,5,8,13,21.....

everything will be very easy

```
class Solution(object):
def climbStairs(self, n):
"""
:type n: int
:rtype: int
"""
if n < 2:
return 1
prev1,prev2 = 0,1
for i in range(n+1):
now = prev1 + prev2
prev2 = prev1
prev1 = now
return now
```