解题思路：

回溯：先序遍历＋路径记录

class Solution {LinkedList<List<Integer>> res = new LinkedList<>();LinkedList<Integer> path = new LinkedList<>();public List<List<Integer>> pathTarget(TreeNode root, int target) {recur(root, target);return res;}void recur(TreeNode root, int tar) {if(root == null) return;path.add(root.val);tar -= root.val;if(tar == 0 && root.left == null && root.right == null)res.add(new LinkedList(path));recur(root.left, tar);recur(root.right, tar);path.removeLast();}
}