二叉树的最大深度

def solve():
    class TreeNode:
        def __init__(self, val=0, left=None, right=None):
            self.val = val
            self.left = left
            self.right = right
    
    # 构建二叉树
    #     3
    #    / \
    #   9  20
    #     /  \
    #    15   7
    root = TreeNode(3)
    root.left = TreeNode(9)
    root.right = TreeNode(20)
    root.right.left = TreeNode(15)
    root.right.right = TreeNode(7)
    
    def max_depth(node):
        if not node:
            return 0
        return max(max_depth(node.left), max_depth(node.right)) + 1
    
    print(max_depth(root))

solve()