129. Sum Root to Leaf Numbers

• categories: Code, Interview Question, leetcode, Medium
• source: https://leetcode.com/problems/sum-root-to-leaf-numbers
• topics: Tree, DFS

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.

Idea

dfs

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def sumNumbers(self, root: Optional[TreeNode]) -> int:
        ans = 0
        def dfs(node, suffix):
            nonlocal ans
            suffix += str(node.val)
            if not node.left and not node.right:
                ans += int(suffix)
                return
            if node.left:
                dfs(node.left, suffix)
            if node.right:
                dfs(node.right, suffix)
        dfs(root, "0")
        return ans