/**
 * 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 {   
    //使用全局变量，避免函数里的传参，节省运行时间
    List<List<Integer>> lists = new ArrayList<>();
    List<Integer> list = new ArrayList<>();
    int target;
    public List<List<Integer>> pathSum(TreeNode root, int target) {
        this.target = target;
        dfs(root, 0);
        return lists;
    }
    void dfs(TreeNode node, int sum){
        if(node == null){     
            return;         
        }
        list.add(node.val);
        sum += node.val;
        if(sum==target && node.left==null && node.right==null){
            //重新开辟空间
            lists.add(new ArrayList(list));
        }
        dfs(node.left, sum);
        dfs(node.right, sum); 
        //回溯  
        list.remove(list.size()-1);         
    }
}