Implement strStr() [LeetCode]

Implement strStr().

Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.

Summary: be careful about the corner case, haystack = "", needle = ""

     char *strStr(char *haystack, char *needle) {
         if(haystack[] == '\0' && needle[] == '\0')
             return haystack;

         int hay_idx = ;
         while(haystack[hay_idx] != '\0'){
             bool find = true;
             int tmp_idx = hay_idx;
             int needle_idx = ;
             while(needle[needle_idx] != '\0'){
                 if(haystack[tmp_idx] == '\0')
                     return NULL;
                 if(haystack[tmp_idx] != needle[needle_idx]){
                     find = false;
                     break;
                 }
                 tmp_idx ++;
                 needle_idx ++;
             }
             if(find)
                 return haystack + hay_idx;
             else
                 hay_idx ++;

         }
         return NULL;
     }