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];

    }