Question Link:

https://leetcode.com/problems/maximum-depth-of-binary-tree/

Simply BFS from root and count the number of levels. The code is as follows.

[-] Python code accepted by LeetCode OJ 
# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return an integer
    def maxDepth(self, root):
        """
        Use BFS from root, and count the steps
        """
        if not root:
            return 0
        count = 0
        q = [root]
        while q:
            count += 1
            new_q = []
            for node in q:
                if node.left:
                    new_q.append(node.left)
                if node.right:
                    new_q.append(node.right)
            q = new_q
        return count