# 104. Maximum Depth of Binary Tree 二叉树的最大深度

@TOC

Total Accepted: 85334 Total Submissions: 188240 Difficulty: Easy

## # 题目描述

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree `[3,9,20,null,null,15,7]`,

``````    3
/ \
9  20
/  \
15   7
``````

return its depth = 3.

## # 解题方法

### # 方法一：BFS

``````# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
def maxDepth(self, root):
"""
:type root: TreeNode
:rtype: int
"""
depth = 0
que = collections.deque()
que.append(root)
while que:
size = len(que)
for _ in range(size):
node = que.popleft()
if not node:
continue
que.append(node.left)
que.append(node.right)
depth += 1
return depth - 1
``````

### # 方法二：DFS

``````# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
def maxDepth(self, root):
"""
:type root: TreeNode
:rtype: int
"""
if not root:
return 0
return 1 + max(self.maxDepth(root.left), self.maxDepth(root.right))
``````

## # 参考资料

559. Maximum Depth of N-ary Treeopen in new window

## # 日期

2015/9/16 10:42:06 2018 年 11 月 ９ 日 —— 睡眠可以