[leetCode刷题笔记]129. Sum Root to Leaf Numbers

public class Solution {    public int sumNumbers(TreeNode root) {       return helper(root, 0);    }    private int helper (TreeNode root, int s) {        if (root == null) return 0;        if (root.left == null && root.right == null) return s * 10 + root.val;        return helper(root.left, 10 * s + root.val) + helper(root.right, 10 * s + root.val);    }}