题目
给定一个二叉树，找出其最大深度。
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。
说明: 叶子节点是指没有子节点的节点。
示例：
给定二叉树 [3,9,20,null,null,15,7]，
3
/ \
9 20
/ \
15 7
返回它的最大深度 3 。
解题思路
递归
# 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: TreeNode) -> int:
return self.countTree(root, 1, 0)
def countTree(self, tree: TreeNode, rank: int, maxRank: int) -> int:
if tree is None: return 0
leftRank = 0
rightRank = 0
if tree is not None:
maxRank = max(rank, maxRank)
if tree.left is not None:
leftRank = self.countTree(tree.left, rank+1, maxRank)
if tree.right is not None:
rightRank = self.countTree(tree.right, rank+1, maxRank)
maxRank = max(maxRank, leftRank, rightRank)
return maxRank
