104. Maximum Depth of Binary Tree

Edited by Charles

Easy · Python · Binary Tree

Problem

Read the original problem on LeetCode.

The official page contains the problem statement, examples, and constraints.

Explanation, examples & other approaches

Core idea

Iteration:

Reference to 102. Binary Tree Level Order Traversal

Original study notes follow, including intermediate snippets and earlier attempts.

Iteration:

1. Reference to 102. Binary Tree Level Order Traversal

# 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
from collections import deque

class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:
        if not root:
            return 0

        dq = deque([root])
        result = 0

        while dq:
            n = len(dq)

            for _ in range(n):
                node = dq.popleft()

                if node.left:
                    dq.append(node.left)
                if node.right:
                    dq.append(node.right)

            result += 1

        return result

Recursion:

1. For every node, the max depth of the current node is the max height from the leaf to this node.
2. In each recursion, make sure the returned value is added by 1.

Study record

Field	Value
Attempts	1
Mastery	❚
The Latest Attempt	January 13, 2026

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 maxDepth(self, root: Optional[TreeNode]) -> int:
        return self.traverse(root)

    def traverse(self, node):
        if not node:
            return 0

        leftHeight = self.traverse(node.left)
        rightHeight = self.traverse(node.right)

        return 1 + max(leftHeight, rightHeight)

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