Triangle

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.

public class Solution {    public int minimumTotal(ArrayList<ArrayList<Integer>> triangle) {        int n = triangle.size();        int [] res = new int [n];        ArrayList<Integer> row = triangle.get(n - 1);        for (int i = 0; i < n; i++) {res[i] = row.get(i);}        for (int i = n -2; i>= 0; --i){        row = triangle.get(i);        for (int j = 0; j <= i ; j++) {res[j] = min(res[j], res[j + 1]) + row.get(j);}        }        return res[0];    }private int min(int a, int b) {return a < b? a : b;}}