编写一个程序，从键盘任意输入一串字符，从屏幕输出并将其存入ex3.txt文件中，并且退出程序后再次运行输入一串字符，上一次输入并存储的字符不被覆盖。(Write a program to input any string of characters from the keyboard, output it from screen and store it in ex3.txt file, and then run a string of characters again after exiting the program. The last input and storage character is not covered.)