leetcode

LeetCode 129. Sum Root to Leaf Numbers

Problem

LeetCode 129. Sum Root to Leaf Numbers

1. 题目简述

给出一棵只含有value值为0-9的二叉树，根节点为root，每一条从root到leaf节点的路径都代表一个数字，求这些所有数字之和。例如：

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.

2. 算法思路

DFS

很简单的DFS的思路，前序遍历的思路，到达根节点时做一次判断。

3. 解法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int res = 0;

    public int sumNumbers(TreeNode root) {
        dfs(root, 0);
        return res;
    }

    private void dfs(TreeNode root, int currSum) {
        if  (root == null) {
            return;
        }
        currSum = currSum * 10 + root.val;
        if (root.left == null && root.right == null) {
            res += currSum;
        }

        dfs(root.left, currSum);
        dfs(root.right, currSum);
    }
}