113. Path Sum II

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    List<List<Integer>> ans = new ArrayList<>();
    public void helper(TreeNode node, int targetSum, List<Integer> path) {
        if(node == null)
            return;
        
        path.add(node.val);
        if(node.left == null && node.right == null) {
            targetSum -= node.val;
            if(targetSum == 0)
                ans.add(new ArrayList<>(path));
        } else {
            helper(node.left, targetSum - node.val, path);
            helper(node.right, targetSum - node.val, path);
        }
        path.remove(path.size()-1);
    }
    public List<List<Integer>> pathSum(TreeNode root, int targetSum) {
        List<Integer> path = new ArrayList<>();
        helper(root, targetSum, path);
        return ans;
    }
}