【LeetCode】120. Triangle (3 solutions)

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.

 

 

使用bottom-up方法将最小值汇聚到root，将中间结果保存在开辟的空间curMin中。

class Solution {
public:
    int minimumTotal(vector<vector<int> > &triangle) {
        int m = triangle.size();
        if(m == 0)
            return 0;
        else if(m == 1)
            return triangle[0][0];
            
        vector<int> curMin = triangle[m-1];
        for(int i = m-2; i >= 0; i --)
        {// for level i
            for(int j = 0; j <= i; j ++)
            {
                curMin[j] = triangle[i][j] + min(curMin[j], curMin[j+1]);
            }
        }
        return curMin[0];
    }
};