leetcode 120. 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.

class Solution {
public:
	int minimumTotal(vector<vector<int>>& triangle) {
		vector<vector<int>>uppersum(triangle.size());
		uppersum[0].push_back(triangle[0][0]);
		for (int i = 1; i < triangle.size(); i++)
		{
			uppersum[i].resize(triangle[i].size());
			uppersum[i][0] = uppersum[i - 1][0] + triangle[i][0];
			uppersum[i].back() = uppersum[i - 1].back() + triangle[i].back();
			for (int j = 1; j < triangle[i].size() - 1; j++)
			{
				uppersum[i][j] = triangle[i][j];
				uppersum[i][j] += uppersum[i - 1][j - 1] < uppersum[i - 1][j] ? uppersum[i - 1][j - 1] : uppersum[i - 1][j];
			}
		}
		int minsum = uppersum.back()[0];
		for (int i = 1; i < triangle.back().size();i++)
		if (uppersum.back()[i] < minsum)
			minsum = uppersum.back()[i];
		return minsum;
	}
};

accepted