[LeetCode] Objective C — Check If a String Has Duplicate Characters

BOOL checkDuplicates(NSString *str)
{
     NSMutableArray *storageArray = [NSMutableArray array];
     for (int i = 0; i < str.length; i++) {
         NSString *charStr = [str substringWithRange:NSMakeRange(i,1)];
         if ([storageArray containsObject: charStr]) {
             return true;
         } else {
             [storageArray addObject: charStr];
         }
     }
     return false;
}