About C language function STRSTR of analysis and implementation

Prototype: char * STRSTR (const char *str1, const char *str2);
# include < String. H >
Find out where the str2 string first appears in the str1 string (not including the str2 string terminator). Returns a pointer to the location, if not found, returns a null pointer.
Returns a pointer to the first occurrence of strSearch in STR, or NULL if strSearch does not appear in STR. IfstrSearch points to a string of zero length, the function Returns STR.

#include <stdio.h>
#include <conio.h>
#include <string.h>
#include <stdlib.h>
#pragma warning (disable:4996)
char *mystrstr(char *s1,char *s2);
int main(void)
{
 char *s="Golden Global View";
 char *l="ob";   //char *l=""
 char *p;
 system("cls");
 p=mystrstr(s,l);
 if (p!=NULL)
 {
  printf("%sn",p);
 }
 else
 {
  printf("Not Found!n");
 }
    getch();
 return 0;
}

char *mystrstr(char *s1,char *s2)
{
 int n;
 if (*s2)                      //So there's two scenarios
 {
        while(*s1)               
  {
            for (n=0;*(s1+n)==*(s2+n);n++)
            {
    if (!*(s2+n+1))            //Find whether the next character is '0'
    {
     return (char*)s1;
    }
            }
   s1++;
  }
  return NULL;
 }
 else
 {
  return (char*)s1;
 }
}

Another implementation:

char *  strstr (buf, sub)
     register char *buf;
     register char *sub;
{
    register char *bp;
    register char *sp;
    if (!*sub)
      return buf;
    while (*buf)
    {
        bp = buf;
        sp = sub;
        do {
            if (!*sp)
              return buf;
        } while (*bp++ == *sp++);
        buf += 1;
    }
    return 0;
}

Another implementation:

#include <iostream>
#include <string>
using namespace std;
//C language implementation STRSTR
const char* isSub(const char* str, const char *subs){
 //A special case
 if(!*subs)
  return str;
 const char* tmp=str;
 while (*tmp!='0')
 {
  //Used to move the parent string backward one character at a time
  const char* tmp1=tmp;
  //Record the substring address
  const char* sub1=subs;
  while (*sub1!='0'&&*tmp1!='0')
  {
   //If it is not equal, it jumps out and moves the parent string back one character
   if (*sub1!=*tmp1)
    break;
   //If the substring is equal and the next character in the substring is the end, then it is a substring of the parent string
   if (*sub1==*tmp1&&*(sub1+1)=='0')
    return tmp;
   //If equal, continue to compare the next character
   if (*sub1==*tmp1)
   {
    sub1++;
    tmp1++;
   }
  }
  tmp++;
 }
 return NULL;
}
int main(){
 char* str1="ababcdddb";
 char* str="";
 const char *res=isSub(str1,str);
 if (res!=NULL)
 {
  cout << res << endl;
 }
 else
  cout << "null" << endl;
 //cout << isSub(str1,str) << endl;
 return 0;
}