#include<iostream>
using namespace std;
char* prefix(char* haystack_start, char* needle_start)
{
char* haystack = haystack_start;
char* needle = needle_start;
while(*haystack && *needle){
if(*(haystack++)!=*(needle++)) return NULL; //填空位置
}
if(*needle) return NULL;
return haystack_start;
}
int main()
{
char *haystack_start="assssw";
char *needle_start="asd";
printf("%s\n",prefix(haystack_start,needle_start));
return 0;
}