简单通信协议：客户端。客户端主要向服务端发送链接请求，然后由用户输入要发送的字符。发送的字符限定在“A”、“C”和“exit”。-Simple communication protocol: the client. The main client sends a link request to the server, and then enter the character to be sent by the user. Send character is defined in the &quo