N-Queens Problem implemented in java

NQueensProblem.java

import java.util.*;

public class NQueensProblem {
	private static int n;
	private static char[][] arr;
	private static LinkedHashSet<String> finalSolution;
	
	static {
		finalSolution = new LinkedHashSet<>();
	}
	
	private static void cleanRow(int i) {
		Arrays.fill(arr[i], ' ');
	}
	
	private static boolean verticalIsSafe(int i , int j) {
		while (i >= 0) {
			if (arr[i--][j] != ' ') {
				return false;
			}
		}
		
		return true;
	}
	
	private static boolean leftDiagonalIsSafe(int i, int j) {
		while (i >= 0 && j >= 0) {
			if (arr[i--][j--] != ' ') {
				return false;
			}
		}
		
		return true;
	}
	
	private static boolean rightDiagonalIsSafe(int i, int j) {
		while (i >= 0 && j < n) {
			if (arr[i--][j++] != ' ') {
				return false;
			}
		}
		
		return true;
	}
	
	private static boolean isSafePosition(int i, int j) {
		return verticalIsSafe(i, j) && leftDiagonalIsSafe(i, j) && rightDiagonalIsSafe(i, j);
	}
	
	private static boolean checkBoard() {
		for (char[] a: arr) {
			boolean emptyRow = true;
			for (char b: a) {
				if (b != ' ') {
					emptyRow = false;
				}
			}
			
			if (emptyRow) {
				return false;
			}
		}
		
		return true;
	}
	
	private static void addSolution(int[] sol) {
		if (checkBoard()) {
			StringBuilder stringBuilder = new StringBuilder();
			HashSet<Integer> hashSet = new HashSet<>();
			for (int a: sol) {
				if (a == -1 || hashSet.contains(a)) {
					return;
				} else {
					hashSet.add(a);
					stringBuilder.append(a);
				}
			}
			
			if (!finalSolution.contains(stringBuilder.toString())) {
				System.out.println(stringBuilder.toString());
				Arrays.stream(arr).forEach(a -> System.out.println(Arrays.toString(a)));
				System.out.println();
			}
			
			finalSolution.add(stringBuilder.toString());
		}
	}
	
	private static void func(int i, int[] sol) {
		if (i != n) {
			for (int j = 0; j < n; j++) {
				if (i == 0) {
					Arrays.fill(sol, -1);
				}
				cleanRow(i);
				
				if (isSafePosition(i, j)) {
					arr[i][j] = 'Q';
					sol[i] = j;
					func(i + 1, sol);
					addSolution(sol);
				}
			}
		}
	}
	
	public static void main(String[] args) {
		n = 5;
		arr = new char[n][n];
		int[] sol = new int[n];
		
		Arrays.stream(arr).forEach(a -> Arrays.fill(a, ' '));
		func(0, sol);
		System.out.println(finalSolution);
		System.out.println(finalSolution.size());
	}
}