Path Sum II

Given a binary tree and a sum, find all root-to-leaf paths where each path's sum equals the given sum.

For example:
Given the below binary tree and sum = 22,

              5
             / \
            4   8
           /   / \
          11  13  4
         /  \    / \
        7    2  5   1

return

[
   [5,4,11,2],
   [5,8,4,5]
]

Solution: DFS + tree traverse.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<ArrayList<Integer>> pathSum(TreeNode root, int sum) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        ArrayList<Integer> subset = new ArrayList<Integer>();
        if(root == null) return result;
        helper(root, sum, result, subset);
        return result;
    }
    public void helper(TreeNode root, int target, ArrayList<ArrayList<Integer>> result, ArrayList<Integer> subset){
        
        if(root.left == null && root.right == null){
            if(root.val == target){ 
                subset.add(root.val); 
                result.add(new ArrayList<Integer>(subset)); // new !!!!!!
                subset.remove(subset.size()-1);
            }
            return;
        }
        subset.add(root.val); 
        if(root.left != null){ helper(root.left, target-root.val, result, subset);}
        if(root.right != null) { helper(root.right, target-root.val, result, subset);}
        subset.remove(subset.size()-1);
    }
}