Leetcode (Python): Triangle

Given a triangle, find the minimum path sum from top to bottom. Each step you may move to adjacent numbers on the row below.

For example, given the following triangle

[
     [2],
    [3,4],
   [6,5,7],
  [4,1,8,3]
]

The minimum path sum from top to bottom is 11 (i.e., 2 + 3 + 5 + 1 = 11).

Note:
Bonus point if you are able to do this using only O(n) extra space, where n is the total number of rows in the triangle.

Solution:

O(1) extra space (using the input as temporary storage):

class Solution:
    # @param triangle, a list of lists of integers
    # @return an integer
    def minimumTotal(self, triangle):
        for i in range(len(triangle)-2,-1,-1):
            for j in range(0,i+1):
                triangle[i][j] += min(triangle[i+1][j],triangle[i+1][j+1])
        return triangle[0][0]