LC129. Sum Root to Leaf Numbers

# 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:
        if not root: return 0
 
        total = 0
 
        def dfs(node, parent_num = 0):
            nonlocal total
 
            if not node:
                return
 
            node_num = parent_num * 10 + node.val
            
            if node.left is None and node.right is None:
                total += node_num
                return
            
            if node.left is not None:
                dfs(node.left, node_num)
            
            if node.right is not None:
                dfs(node.right, node_num)
        
        dfs(root)
 
        return total

Complexity

Time: $O(n)$
Space: $O(n)$