public static Board getPrev(Board currentBoard){
		Board previousBoard = new Board();
		if (history.size() == 0){
			return currentBoard;
		}
		Object[][] deltaBoard = history.get(history.size() - 1);
		for (int y = 0;y < 8;y++){
			for (int x = 0;x < 8;x++){
				if (deltaBoard[y][x] instanceof Integer){
					if (((Integer)deltaBoard[y][x]) == 0){
						previousBoard.set(x, y, null);
					}
					else{
						previousBoard.set(x, y, currentBoard.get(x,y));
					}
				}
				else{
					previousBoard.set(x, y, (ChessPiece)deltaBoard[y][x]);
				}
			}
		}
		return previousBoard;
	}