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- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
    const NSInteger kItemNumOfRow = 3;
    UICollectionViewFlowLayout *layout = (UICollectionViewFlowLayout *)collectionViewLayout;
    NSInteger row = indexPath.row;
    CGFloat totalWidth = collectionView.frame.size.width - layout.sectionInset.left - layout.sectionInset.right - layout.minimumInteritemSpacing * (kItemNumOfRow - 1);
    CGFloat width;
    
    if (row % kItemNumOfRow) {
        width = roundf(totalWidth / kItemNumOfRow);
    } else {
        width = totalWidth - roundf(totalWidth / kItemNumOfRow) * (kItemNumOfRow - 1);
    }
    
    return CGSizeMake(width, width);
}