查找子字符串，并记录查找的第一个子字符串的位置
code： https://github.com/lujinjianst/myNCCL/blob/master/exercise/exercise-12.3.c
computational results ：
[haoyue@centos exercise]$ ./a.out
input string s1 and s2:
hello
helloworld
helloworld is not in hello
[haoyue@centos exercise]$ ./a.out
input string s1 and s2:
world hello
hello
hello is in world hello ,the first position 6
[haoyue@centos exercise]$ ./a.out
input string s1 and s2:
helloworldhello
hello
hello is in helloworldhello ,the first position 0
以上程序存在一些弊端，大家可以进一步研究。。。
