leetcode第31題（triangle）

題目：

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. 

思路：可以發現第l排的第k個元素的下一排只可能是第l+1排的第k個元素或者是第l+1排的第k+1個元素。遞歸即可實現

代碼：

import java.util.ArrayList;

public class Solution {
    public int minimumTotal(ArrayList<ArrayList<Integer>> triangle) {
        int sum;
        sum = Result(triangle,0,0);
        return sum;
    }
    public int Result(ArrayList<ArrayList<Integer>> triangle,int l ,int k){
        int sum = triangle.get(l).get(k);
        if(l<triangle.size()-1){
            sum = sum+Math.min(Result(triangle,l+1,k),Result(triangle,l+1,k+1));
        }
        return sum;
    }
}