why my dynamic prototypes cell overlapping?

Question

When I use dynamic prototypes, I found my label is overlapping.

"firstLabel" is the default text of the label I have inserted on storyboard and I set the text "ios" when the program run.

Following are my code.

@implementation ViewController{
    NSArray* _books;
}
- (void)viewDidLoad {
    _books=@[@"ios",@"android"];
    self.table.dataSource=self;
    self.table.delegate=self;
    // Do any additional setup after loading the view, typically from a nib.
}

- (UITableViewCell*)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
    NSInteger row=indexPath.row;
    NSString* cellID=(row%2==0?@"cell1":@"cell2");
    UITableViewCell* cell=[tableView dequeueReusableCellWithIdentifier:cellID forIndexPath:indexPath];
    UILabel* label=(UILabel*)[cell viewWithTag:1];
    label.text=_books[row];
    return cell;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section{
    return _books.count;
}

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath{
    return 80;
}

Answer 1

Thank everyone for your replies. I found when I use two cellIDs, they maybe mixed. After I set only one cellID, it gets right.

NSString* cellID=(row%2==0?@"cell1":@"cell2");

change to NSString* cellID=@"cell1";

Answer 2

because the firstLabel's height is equal to the cell. and the firstLabel in the above of the custom label.