【LeetCode】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) {
        if(triangle.size()==0)
            return 0;
        int len = triangle.size();
        int[] re = new int[len];
        re[0]=triangle.get(0).get(0);
        int min=re[0];
        for(int i=1;i<triangle.size();i++){
            ArrayList<Integer> al = triangle.get(i);
            int pre = 0;
            
            for(int j=0;j<al.size();j++){
                if(j==0){
                    pre=re[0];
                    re[0]+=al.get(j);
                    min=re[0];
                    continue;
                }
                if(j==al.size()-1){
                    re[j]=pre+al.get(j);
                    if(re[j]<min)
                        min=re[j];
                    continue;
                }
                int temp = re[j];
                re[j]=Math.min(pre, re[j])+al.get(j);
                pre = temp;
                if(re[j]<min)
                    min=re[j];
            }
            
        }
        return min;
        
    }
}