070.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
        """
        if n <= 1:
            return 1
        pre, ppre = 1, 1
        for i in xrange(2, n + 1):
            tmp = pre
            pre = ppre + pre
            ppre = tmp
        return pre