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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(triangle == null) return 0;
        if(triangle.size() == 0) return 0;
        int[] a = new int[triangle.size()];
        for(int i = 0; i < triangle.size(); i ++){
            a[i] = triangle.get(triangle.size() - 1).get(i);
        }
        for(int j = triangle.size() - 1; j > 0; j --){
            for(int i = 0; i < j; i ++){
                a[i] = triangle.get(j - 1).get(i) + Math.min(a[i],a[i+1]);
            }
        }
        return a[0];
    }
}

第二遍：

public class Solution {
    public int minimumTotal(ArrayList<ArrayList<Integer>> triangle) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        Integer[] a = triangle.get(triangle.size()-1).toArray(new Integer[0]);
        for(int j = triangle.size() - 1; j > 0; j --)
            for(int i = 0; i < j; i ++)
                a[i] = triangle.get(j-1).get(i) + Math.min(a[i],a[i+1]);
        return a[0];
    }
}