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Leetcode 104 二叉树的最大深度

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给定一个二叉树，找出其最大深度。

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# 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

# DFS
class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:
        def helper(root, depth):
            if not root:
                return depth
            else:
                return max(helper(root.left, depth+1), helper(root.right, depth+1))
        return helper(root, 0)

# BFS
class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:
        if not root: 
            return 0
        queue, res = [root], 0
        while queue:
            tmp = []
            for node in queue:
                if node.left:
                    tmp.append(node.left)
                if node.right:
                    tmp.append(node.right)
            queue = tmp
            res += 1
        return res