表格视图单元格选中标记在滚动时更改位置

I am new to Swift. I have created a simple list in a tableview. When the user long presses on a row, that row will get checked. It's working perfectly fine. But when I scroll down, check mark changes its position. I also tried to store position in NSMutableSet. But still it's not working. Maybe I am doing something wrong.I am new to Swift. I have created a simple list