[Leetcode] 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.

Analysis:

class Solution {
public:
    int minimumTotal(vector<vector<int> > &triangle) {
        vector<int> ret(triangle.back().size() + 1, 0);
        for(int i=triangle.size()-1; i>=0; i--)
        {
            for(int j = 0; j < triangle[i].size(); j++)
            {
                ret[j] = min(ret[j], ret[j+1]) + triangle[i][j]; 
            }
        }
        
        return ret[0];
    }
};