Leetcode 129. Sum Root to Leaf Numbers

题目链接

https://leetcode.com/problems/sum-root-to-leaf-numbers/description/

题目描述

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   3
Output: 25
Explanation:
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   1
Output: 1026
Explanation:
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.

题解

本题的思路比较简单，使用递归的方式。遍历一个节点，就累加计数，当到达根节点的时候，就把累加结果返回。

代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int sum = 0;
    public int sumNumbers(TreeNode root) {
        sumNumbers(root, 0);
        return sum;
    }
    public void sumNumbers(TreeNode root, int val) {
        if (root == null) {
            return ;
        }
        if (root.left == null && root.right == null) {
            sum += val * 10 + root.val;
            return ;
        }
        sumNumbers(root.left, val * 10 + root.val);
        sumNumbers(root.right, val * 10 + root.val);
    }
}