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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.

返回树的深度，可以用DFS或者BFS算法，都很简单.

DFS:用maxdepth记录最大深度

class Solution:    def maxDepth(self, root: TreeNode) -> int:        if root is None: return 0        stack=[(root,1)]        maxdepth=1        while stack:            node,depth = stack.pop()            if depth>maxdepth:                maxdepth=depth            if node.left:                stack.append((node.left, depth+1))            if node.right:                stack.append((node.right, depth+1))        return maxdepth

BFS：

class Solution(object):    def maxDepth(self, root):        """        :type root: TreeNode        :rtype: int        """        depth = 0        level = [root] if root else []        while level:            depth += 1            queue = []            for el in level:                if el.left:                    queue.append(el.left)                if el.right:                    queue.append(el.right)            level = queue                    return depth

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