104. 二叉树的最大深度

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]

点击查看DFS的代码

class Solution:
    def maxDepth(self, root) -> int:
        if not root:
            return 0
        # 利用定义，计算左右子树的最大深度
        left = self.maxDepth(root.left)
        right = self.maxDepth(root.right)
        # 整棵树的最大深度等于左右子树的最大深度取最大值，然后再加上根节点自己
        return max(left, right) + 1

点击查看BFS的代码

class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        if root is None:
            return 0
        q = deque([root])
        level = 0
        while q:
            n = len(q)
            for i in range(n):
                node = q.popleft()
                if node.left is not None:
                    q.append(node.left)
                if node.right is not None:
                    q.append(node.right)
            level += 1
        return level

点击查看pycharm-DFS代码

from collections import deque


class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None


class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        if root is None:
            return 0
        q = deque([root])
        level = 0
        while q:
            n = len(q)
            for i in range(n):
                node = q.popleft()
                if node.left is not None:
                    q.append(node.left)
                if node.right is not None:
                    q.append(node.right)
            level += 1
        return level


if __name__ == '__main__':
    # 创建根节点
    root = TreeNode(3)
    # 插入左右子节点
    root.left = TreeNode(9)
    root.right = TreeNode(20)
    root.right.left = TreeNode(15)
    # 创建一个 Solution 对象
    solution = Solution()
    # 计算整棵树的
# 计算整棵树的最大深度
    depth = solution.maxDepth(root)
    print(depth)

点击查看pycharm-BFS代码

from collections import deque


class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None


class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        if root is None:
            return 0
        q = deque([root])
        level = 0
        while q:
            n = len(q)
            for i in range(n):
                node = q.popleft()
                if node.left is not None:
                    q.append(node.left)
                if node.right is not None:
                    q.append(node.right)
            level += 1
        return level


if __name__ == '__main__':
    # 创建根节点
    root = TreeNode(3)
    # 插入左右子节点
    root.left = TreeNode(9)
    root.right = TreeNode(20)
    root.right.left = TreeNode(15)
    # 创建一个 Solution 对象
    solution = Solution()
    # 计算整棵树的
# 计算整棵树的最大深度
    depth = solution.maxDepth(root)
    print(depth)

总结

Python中collections模块（高性能集合操作）
deque([list]).popleft()：弹出list最左侧的元素；