【Leetcode】113. 路径总和II

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

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

public class Solution {
    public List<List<Integer>> pathSum(TreeNode root, int sum) {
        List<List<Integer>> res = new LinkedList<>();
        helper(root, sum, res, new LinkedList<>());
        return res;
    }

    public void helper(TreeNode root, int sum, List<List<Integer>> res, List<Integer> current) {
        if (root == null) {
            return;
        }
        current.add(root.val);
        if (root.left == null && root.right == null && sum == root.val) {
            // leaf node.
            res.add(new LinkedList<>(current));
            // back track.
            current.remove(current.size() - 1);
            return;
        }

        helper(root.left, sum - root.val, res, current);
        helper(root.right, sum - root.val, res, current);
        // back track.
        current.remove(current.size() - 1);
    }
}