[leetcode] 104. Maximum Depth of Binary Tree

Given the root of a binary tree, return its maximum depth.

A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

 

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: 3

Example 2:

Input: root = [1,null,2]
Output: 2

 

Constraints:

• The number of nodes in the tree is in the range [0, 104].
• -100 <= Node.val <= 100

 

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root is None:
            return 0 

        queue = collections.deque([root])
        depth = 0 

        while queue:
            depth += 1 
            # 큐 연산 추출 노드의 자식 노드 삽입 
            for _ in range(len(queue)):
                cur_root = queue.popleft() 
                if cur_root.left:
                    queue.append(cur_root.left)
                if cur_root.right:
                    queue.append(cur_root.right)

        # BFS 반복 횟수 == 깊이 
        return depth