字符串指针，以探讨指针如何工作

代码说明：

String Pointers to explore how the pointer work -The program takes lines from standard input and keeps the last n of them in memory as it goes through standard input. When it gets to an EOF, it prints the last n lines out. You may assume n is less than 2000 and each individual line is not longer than 1000 including the newline and the null terminator.