LeetCode Offer 55 I. Depth of Binary Tree

class Solution:  # recursion
  def maxDepth(self, root: TreeNode) -> int:
    def dfs(node):
      if not node:
        return 0
      left_depth = dfs(node.left)
      right_depth = dfs(node.right)
      return max(left_depth, right_depth) + 1
    return dfs(root)

class Solution:  # short form recursion
  def maxDepth(self, root: TreeNode) -> int:
    return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1 if root else 0

from collections import deque

class Solution:  # level-order traversal
  def maxDepth(self, root: TreeNode) -> int:
    if not root: return 0
    ans = 0
    queue = deque([root])
    while queue:
      ans += 1
      new_queue = deque()
      for node in queue:
        if node.left: new_queue.append(node.left)
        if node.right: new_queue.append(node.right)
      queue = new_queue
    return ans