LeetCode triangle

题目描述

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 is11(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.

解题思路： 动态规划的思想，从下往上，记录每个位置最短的路径和

import java.util.*;
public class Solution {
    public int minimumTotal(ArrayList<ArrayList<Integer>> triangle) {
        int rows = triangle.size();
		for (int i = rows - 2; i >= 0; i--) {
			for (int j = 0; j <= i; j++) {
				int cur = triangle.get(i).get(j);
				int nextLeft = triangle.get(i+1).get(j);
				int nextRight = triangle.get(i+1).get(j+1);
				triangle.get(i).set(j, Math.min(nextLeft, nextRight) + cur);
			}
		}
		return triangle.get(0).get(0);   
    }
}