《c prime plus》课后一习题，大神指点下？

xiaowh00

#include <stdio.h>
#include <string.h>

#define MAX 100
#define STOP "quit"

// Write a function called string_in() that takes two string pointers as arguments. If the second string is contained in the first string, have the function return the address at which the contained string begins. For instance, string_in("hats", "at" would return the address of the a in hats. Otherwise, have the function return the null pointer. Test the function in a complete program that uses a loop to provide input values for feeding to the function.


char *string_in(char *,char *);

int main(void)
{
        char s1[MAX],s2[MAX];
        char *ptr;

        printf("Input the first string(less than %d quit to quit):\n",MAX);
        gets(s1);
        while(strcmp(s1,STOP)!=0)
        {
                printf("Input the second string:\n";
                gets(s2);
                ptr=string_in(s1,s2);       
                if(ptr)
                {
                        printf("The address's of %s is %p\n",s1,s1);
                        printf("The first occurrence of %s in %s is %p\n",s2,s1,ptr);       
                }
                else
                        printf("There is no %s in %s\n",s2,s1);

                printf("Input the first string(less than %d quit to quit):\n",MAX);
                gets(s1);
        }
        return 0;
}

char *string_in(char *s1,char *s2)
{
        char *pt1,*pt2;
        pt1=s1;
        pt2=s2;

        for(;*pt1 && *pt2
        {
                if(*pt1==*pt2)
                {
                        pt1++;
                        pt2++;
                }
                else
                {
                        pt1++;
                        pt2=s2;
                }
        }
        if(! *pt2)
                return pt1-strlen(s2);
        else
                return NULL;
}

输入两个字符串，找出第二个字符串在第一个字符串中第一次出现的位置，我初步测试感觉是满足题目的，但是感觉有点乱，望大神指点下？