104.二叉树的最大深度.py

#
# @lc app=leetcode.cn id=104 lang=python3
#
# [104] 二叉树的最大深度
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# https://leetcode-cn.com/problems/maximum-depth-of-binary-tree/description/
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# algorithms
# Easy (72.13%)
# Likes:    480
# Dislikes: 0
# Total Accepted:    137.6K
# Total Submissions: 189.9K
# Testcase Example:  '[3,9,20,null,null,15,7]'
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# 给定一个二叉树，找出其最大深度。
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# 二叉树的深度为根节点到最远叶子节点的最长路径上的节点数。
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# 说明: 叶子节点是指没有子节点的节点。
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# 示例：
# 给定二叉树 [3,9,20,null,null,15,7]，
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# ⁠   3
# ⁠  / \
# ⁠ 9  20
# ⁠   /  \
# ⁠  15   7
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# 返回它的最大深度 3 。
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#

# @lc code=start
# 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:
        # # 1. DFS 递归 O(N) O(N)
        # if not root:
        #     return 0
        # return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1
        
        # 2. BFS 迭代 O(N) O(N)
        if not root:
            return 0
        depth = 0
        q = [root]
        while q:
            size  = len(q)
            for _ in range(size):
                node = q.pop(0)
                if node.left:
                    q.append(node.left)
                if node.right:
                    q.append(node.right)
            depth += 1
        return depth

# @lc code=end