//计算树的最大深度
    func p_maxDepth(root: TreeNode?) -> Int {
        
        guard let root = root else {
            return 0
        }
        return max(p_maxDepth(root: root.left), p_maxDepth(root: root.right)) + 1
    }
    
    

@objc func maxDepth()
    {
        //给定二叉树 [3, 9, 20, nil, nil, 15, 7]  深度是3
        
        let root = TreeNode.init(val: 3)
        root.left = TreeNode.init(val: 9)
        root.right = TreeNode.init(val: 20)
        let second = root.right
        second?.left = TreeNode.init(val: 10)
        second?.right = TreeNode.init(val: 7)
        
        let result = self.p_maxDepth(root: root)
        
         print("二叉树的深度\(result)")
        
    }