字符串的首地址就是指针
char * sub_str(char *s1,char *s2,int m,int n){int i;// char *p = s2;s1=&s1[m-1];s2=&s2[0];for(i=0;i *(s2+i)=*(s1+i); *(s2+n)='\0';//return p;}
