113. 路径总和 II

示例:
给定如下二叉树，以及目标和 sum = 22，

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

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        if (root == null) {
            return res;
        }
        dfs(root, sum, res, list);
        return res;
    }

    private void dfs(TreeNode root, int sum, List<List<Integer>> res, List<Integer> list) {
        if (root == null) {
            return;
        }
        list.add(root.val);
        if (root.val == sum && root.left == null && root.right == null) {
            res.add(new ArrayList<>(list));
        }
        dfs(root.left, sum - root.val, res, list);
        dfs(root.right, sum - root.val, res, list);
        list.remove(list.size() - 1);
    }
}