O(n)的空間，從底部開始

class Solution(object):
    def minimumTotal(self, triangle):
        """
        :type triangle: List[List[int]]
        :rtype: int
        """
        n = len(triangle)
        dp=triangle[n-1]
        for i in range(n-2,-1,-1):
            for j in range(len(triangle[i])):
                dp[
 
j]=triangle[i][j]+min(dp[j],dp[j+1])
        return dp[0]