deque in Python

queue_bfs.py

# 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:
        depth = 0
        queue = deque([root])
        while queue:
            next_layer = 0
            for _ in range(len(queue)):
                node = queue.popleft()
                if node:
                    next_layer = 1
                    queue.append(node.left)
                    queue.append(node.right)
            if next_layer:
                depth += 1
        return depth