129.Sum Root to Leaf Numbers
===

###### tags: `Medium`,`Tree`,`DFS`

[129. Sum Root to Leaf Numbers](https://leetcode.com/problems/sum-root-to-leaf-numbers/)

### 題目描述

You are given the `root` of a binary tree containing digits from `0` to `9` only.

Each root-to-leaf path in the tree represents a number.

* For example, the root-to-leaf path 1 -> 2 -> 3 represents the number 123.

Return *the total sum of all root-to-leaf numbers. Test cases are generated so that the answer will fit in a **32-bit** integer.*

A **leaf** node is a node with no children.

### 範例

**Example 1:**


Input: root = [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: root = [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.

**Constraints**:

* The number of nodes in the tree is in the range `[1, 1000]`.
* 0 <=` Node.val` <= 9
* The depth of the tree will not exceed `10`.

### 解答
#### Python

class Solution:
    def sumNumbers(self, root: Optional[TreeNode]) -> int:
        nums = []

        def dfs(root, num=None):
            if not root: return
            if not num: num = []
            
            num.append(str(root.val))
            if not root.left and not root.right:
                num_str = "".join(num)
                nums.append(int(num_str))

            dfs(root.left, num)
            dfs(root.right, num)

            num.pop()

        dfs(root)
        return sum(nums)

> [name=Ron Chen][time=Tue, Mar 14, 2023]

### Reference

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