triangle leetcode C++

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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 is11(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.

C++

class Solution {
public:
    int minimumTotal(vector<vector<int> > &triangle) {
        int n = triangle.size();
        vector<int> dp(triangle.back());
        for (int i = n - 2; i >= 0; --i)
            for (int j = 0; j <= i; ++j)
                dp[j] = triangle[i][j] +  min(dp[j],dp[j+1]);
        return dp[0];
    }
    int minimumTotal2(vector<vector<int>>& triangle){
        int len = triangle.size();
        vector<int> dp(len,0);
        for(int i = len - 1; i >= 0; --i){
            for(int j = 0; j <= i; ++j){
                if(i == len - 1) dp[j] = triangle[i][j];
                else dp[j] = triangle[i][j] + min(dp[j],dp[j+1]);
            }
        }
        return dp[0];
    }
};