LeetCode--triangle

题目描述

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.

分析：动态规划题，从下向上

class Solution {
public:
    int minimumTotal(vector<vector<int> > &triangle) {
        vector<int> res(triangle.back());
        //用一个一维数组，以triangle最后一行元素开始，逐层向上叠加覆盖
        for(int i=triangle.size()-2;i>=0;--i){
            for(int j=0;j<triangle[i].size();++j){
                res[j]=triangle[i][j]+min(res[j],res[j+1]);
                //向上叠加，上一层等于下一层的正下方和右邻近点
            }
        }
        return res[0];
    }
};