Min Cost Climbing Stairs — Python

# Time:  O(n)
# Space: O(1)

class Solution(object):
    def minCostClimbingStairs(self, cost):
        """
        :type cost: List[int]
        :rtype: int
        """
        dp = [0] * 3
        for i in reversed(xrange(len(cost))):
            dp[i%3] = cost[i] + min(dp[(i+1)%3], dp[(i+2)%3])
        return min(dp[0], dp[1])