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