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a b c d  d  d a d a c a b d a d d d a d c k
             ->
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char* strstr(const char* s1, const char* s2)
{
      if(s1 == NULL || s2 == NULL)
           return NULL;
      while(*s1)
      {  
           while(1)
         {
               int i = 0;
               if(s2[i]  == NULL)
                    return (char*)s1;
               if(s2[i] != s1[i])
                    break;
               i++;
         }
         s1++;
     }
      return NULL;
}