剑指 Offer 34. 二叉树中和为某一值的路径

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    LinkedList<List<Integer>> res = new LinkedList<>();
    LinkedList<Integer> path = new LinkedList<>();
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        helper(root, sum);
        return res;
    }
    void helper(TreeNode root, int sum){
        if (root == null)
            return;
        path.add(root.val);
        sum -= root.val;
        if (sum == 0 && root.left == null && root.right == null)
            res.add(new LinkedList(path));
        helper(root.left, sum);
        helper(root.right, sum);
        path.removeLast();
    }
}