Given an integer n, return the number of structurally unique **BST’**s (binary search trees) which has exctly n nodes unique values from 1 to n.

给你一个证书n，求恰由n个节点组成且节点值从1到n互不相同的二叉搜索树有多少种？返回满足题意的二叉搜索树的种数。

¶Digital Reform

There was a vary circumstances from digital governance, industry, manufacture and medicine, and the booming of
new business models has stimulated enterprises to accelerate digital transformation for better company management
and customer service, which further promotes the upgrading and optimization of the Chinese digital economy.

¶Introduction

Keys are critical in cryptographic operations in public-key infrastructure(PKI). SSL handshake invoke exchanging several pieces of information, such as public keys, private keys, shared secrets, pre-master secrets, and master secrets.

Let’s explore what happens between the key exchange and the data encryption and authentication.

本章覆盖有：

• 简单、自由(free)函数和方法需要避免写入生命周期指示器(lifetime specifiers)，因为它们是被推断的
• 为什么包含引用的结构体(structs)、元组-结构体(tuple-structs)、枚举(enums)需要生命周期指示器(lifetime specifiers)
• 如何为结构体(structs)、元组-结构体(tuple-structs)、枚举(enums)编写生命周期指示器(lifetime specifiers)
• 为什么包含指向泛型参数的结构体需要生命周期边界(协变、逆协变)