Sum Root to Leaf Numbers — LeetCode 129 Python Solution

The problem

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.

Example

Input

root = [1,2,3]

Output

25

Explanation

The root-to-leaf path 1->2 represents the number 12.

Python solution

# 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:
        def dfs(root, s):
            if root is None:
                return 0
            s = s * 10 + root.val
            if root.left is None and root.right is None:
                return s
            return dfs(root.left, s) + dfs(root.right, s)

        return dfs(root, 0)

Complexity

Pattern: Tree Traversal

Choose the order — preorder, inorder, postorder, level — and the problem solves itself. LeetCode 129. Sum Root to Leaf Numbers is filed here because LeetCode tags it Tree and Binary Tree, which is the vocabulary this hub collects.

The tree traversal guide has the Python template for the pattern and the 225 LeetCode problems that use it.

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