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[Leetcode]129. Sum Root to Leaf Numbers(C++)

题目描述

题目链接:129. Sum Root to Leaf Numbers

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.

例子

例子 1

Input: [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.

例子 2

Input: [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.

Note

A leaf is a node with no children.

解题思路

采用 dfs 遍历二叉树,传入当前路径的和,执行以下操作:

  • 如果节点为空,直接返回
  • 否则,将当前值先乘 10,再加上当前节点值
  • 如果当前节点是叶节点,则将当前路径和加入结果中
  • 否则对左右子树递归执行操作

代码如下:

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class Solution {
public:
int sumNumbers(TreeNode* root) {
int result = 0;
dfs(root, 0, result);
return result;
}

private:
void dfs(TreeNode* root, int current_sum, int& result) {
if (!root) return;
current_sum *= 10;
current_sum += root->val;
if (!root->left && !root->right) {
result += current_sum;
}

dfs(root->left, current_sum, result);
dfs(root->right, current_sum, result);
}
};
  • 时间复杂度: O(n)
  • 空间复杂度: O(h)

GitHub 代码同步地址: 129.SumRootToLeafNumbers.cpp

其他题目:
GitHub: Leetcode-C++-Solution
博客: Leetcode-Solutions