Triangle——LeetCode

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.

这个题跟之前的这道题几乎是一样的。之前做的用的二维数组保存的，其实由底向上计算，只需要一维数组即可。

    public int minimumTotal(List<List<Integer>> triangle) {
        if(triangle==null||triangle.size()==0){
            return 0;
        }
        if(triangle.size()==1){
            return triangle.get(0).get(0);
        }
        int[] res = new int[triangle.size()];
        for(int i=0;i<triangle.size();i++){
            res[i]=triangle.get(triangle.size()-1).get(i);
        }
        for(int i=triangle.size()-2;i>=0;i--){
            List<Integer> row = triangle.get(i);
            for(int j = 0;j<row.size();j++){
                res[j]=Math.min(res[j],res[j+1])+row.get(j);
            }
        }
        return res[0];
    }