LeetCode : 120. Triangle三角形最短路径

川长思鸟来 2021-06-12 20:36 468阅读 0赞

**试题：**  
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.  
**代码：**

import java.util.*;
    
    class Solution {
        public int minimumTotal(List<List<Integer>> triangle) {
            if(triangle == null || triangle.size() == 0 || triangle.get(0) == null || triangle.get(0).size() == 0) return 0;
            int maxl = triangle.get(triangle.size()-1).size();
            int[][] dp = new int[2][maxl];
            int flag = 1;
            Arrays.fill(dp[0], Integer.MAX_VALUE);
            Arrays.fill(dp[1], Integer.MAX_VALUE);
            dp[0][0] = triangle.get(0).get(0);
            
            for(int i = 1; i < triangle.size(); i++){
                List<Integer> temp = triangle.get(i);
                for(int j = 0; j < temp.size(); j++){
                    if(j == 0){
                        dp[flag][j] = dp[1-flag][j] + temp.get(j);
                    }else if(j == temp.size()-1){
                        dp[flag][j] = dp[1-flag][j-1] + temp.get(j);
                    }else{
                        dp[flag][j] = Math.min(dp[1-flag][j], dp[1-flag][j-1]) + temp.get(j);
                    }
                }
                flag = 1 - flag;
            }
            int min = Integer.MAX_VALUE;
            for(int num : dp[1 - flag]){
                min = Math.min(min, num);
            }
            return min;
        }
    }