Android listview items shuffling when scrolling from top to bottom?

public class CustomAdapter extends BaseAdapter
{
	LayoutInflater mInlfater;
	ArrayList<hashmap><string,string>> list;
	String rno="",name="";
	public CustomAdapter(Context context,ArrayList<hashmap><string,string>> list) 
	{
		mInlfater = LayoutInflater.from(context);
		this.list =list;
	}
	@Override
	public int getCount() 
	{
		return list.size();

	}

	@Override
	public Object getItem(int position) 
	{	
		return position;
	}

	@Override
	public long getItemId(int position)
	{	
		return position;
	}

	@Override
	public View getView(final int position, View convertView, ViewGroup parent) 
	{
		final ViewHolder holder;
		if(convertView ==null)
		{
			convertView = mInlfater.inflate(R.layout.attendancelistview,null);
			holder = new ViewHolder();
			holder.b1 = (Button)convertView.findViewById(R.id.row3);
			holder.tv1 = (TextView)convertView.findViewById(R.id.row1);
			holder.tv2 = (TextView)convertView.findViewById(R.id.row2);
		   convertView.setTag(holder);
		}
		else
		{
			holder =(ViewHolder) convertView.getTag();
		}
		HashMap<string,string> map = list.get(position);
		holder.tv1.setText(map.get("Rno"));
		holder.tv2.setText(map.get("StudentName"));
		holder.b1.setOnClickListener(new OnClickListener()
		{

			@Override
			public void onClick(View v)
			{
				if(holder.b1.getText().equals("Present"))
				{
					holder.b1.setText("Absent");
					list.get(position).put("Attendance", "Absent");
					holder.b1.setTextColor(Color.RED);

				}
				else if(holder.b1.getText().equals("Absent"))
				{
					holder.b1.setText("Present");
					list.get(position).put("Attendance", "Present");
					holder.b1.setTextColor(Color.GREEN);

				}
				else
				{
					holder.b1.setText("Present");
					list.get(position).put("Attendance", "Present");
					holder.b1.setTextColor(Color.GREEN);
				}
				String rno1=  String.valueOf(holder.tv1.getText());
				name=String.valueOf(holder.tv2.getText());
			}

		});
		return convertView;
	}
	static class ViewHolder
	{
		Button b1;
		TextView tv1,tv2,tv3;
	}

}