[leetcode]Triangle

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.

算法思路：

思路1：

自上而下，用一个数组记录从根节点到第 i 层的所有节点的加权之后的情况，跌倒到最后一层时，即可求出最短路径。需要O(n)空间。

不再实现

 

思路2：

    public int minimumTotal(List<List<Integer>> triangle) {
        if(triangle == null || triangle.size() == 0) return 0;
        int height = triangle.size();
        for(int i = height - 2; i >= 0; i--){
            List<Integer> list = triangle.get(i);
            for(int j = 0; j < list.size();j++){
                int min = triangle.get(i + 1).get(j) < triangle.get(i + 1).get(j + 1) ? triangle.get(i + 1).get(j) : triangle.get(i + 1).get(j + 1);  
                list.set(j, list.get(j) + min);
            }
        }
        return triangle.get(0).get(0);
    }

这代码是之前看到的，很厉害。