Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.
An example is the root-to-leaf path 1->2->3 which represents the number 123.
Find the total sum of all root-to-leaf numbers.Note: A leaf is a node with no children.Example:Input: [1,2,3] 1 / \ 2 3Output: 25Explanation:The root-to-leaf path 1->2 represents the number 12.The root-to-leaf path 1->3 represents the number 13.Therefore, sum = 12 + 13 = 25.
Example 2:
Input: [4,9,0,5,1] 4 / \ 9 0 / \5 1Output: 1026Explanation:The root-to-leaf path 4->9->5 represents the number 495.The root-to-leaf path 4->9->1 represents the number 491.The root-to-leaf path 4->0 represents the number 40.Therefore, sum = 495 + 491 + 40 = 1026.
难度：medium
题目：给定结点值只包含0-9的二叉树，每条从根到叶子的路径表示一个整数。找出所有这样的数并返回其和。
注意：叶结点即没有左右子结点思路：前序遍历
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */class Solution { public int sumNumbers(TreeNode root) { return sumNumbers(root, ""); } private int sumNumbers(TreeNode root, String s) { if (null == root) { return 0; } s += root.val; if (null == root.left && null == root.right) { return Integer.parseInt(s); } return sumNumbers(root.left, s) + sumNumbers(root.right, s); }}