随即生成任意规则的几个字符。

NSMutableSet *mySet = [[NSMutableSet alloc]init];
        NSArray *myArray = [[NSArray alloc]initWithObjects:@"0",@"1",@"2",@"3",@"4",@"5",@"6",@"7",@"8",@"9",@"a",@"b",@"c",@"d",@"e",@"f",@"g",@"h",@"i",@"j",@"k",@"l",@"m",@"n",@"o", nil];
        for (int i =0; i<4000; i++) {
            int x1 = arc4random() % 25;
            int x2 = arc4random() % 25;
            int x3 = arc4random() % 25;
            int x4 = arc4random() % 25;
            int x5 = arc4random() % 25;
           
            NSString *a1 = [myArray objectAtIndex:x1];
            NSString *a2 = [myArray objectAtIndex:x2];
            NSString *a3 = [myArray objectAtIndex:x3];
            NSString *a4 = [myArray objectAtIndex:x4];
            NSString *a5 = [myArray objectAtIndex:x5];
//            NSLog(@"%@%@%@%@%@ \n",a1,a2,a3,a4,a5);
            NSString *tempString = [NSString stringWithFormat:@"%@%@%@%@%@",a1,a2,a3,a4,a5];
            [mySet addObject:tempString];
        }
        NSLog(@"mySet == %@",mySet);
        NSLog(@"mySet.count===%ld",(unsigned long)mySet.count);