NQueenII.java

public class Solution {

    private Set<Integer> usedColIndex = new HashSet<Integer>();
    private Set<Integer> used135DegreeDiagonal = new HashSet<Integer>();
    private  Set<Integer> used45DegreeDiagonal = new HashSet<Integer>();
    private int count = 0;

    public int totalNQueens(int n) {
        helper(n,0);
        return count;
    }
    public void helper( int n, int row) {
        if (row == n) {
            count++;
            return;
        } else {
            for (int col = 0; col < n; col++) {
                if (isValid(row, col)) {
                    usedColIndex.add(col);
                    used135DegreeDiagonal.add(col + row);
                    used45DegreeDiagonal.add(row - col);
                    helper(n, row + 1);
                    usedColIndex.remove(col);
                    used135DegreeDiagonal.remove(col + row);
                    used45DegreeDiagonal.remove(row - col);
                }
            }
        }

    }

    private boolean isValid(int row, int col){
        if(usedColIndex.contains(col) || used135DegreeDiagonal.contains(col+row) || used45DegreeDiagonal.contains(row-col)) {
            return false;
        } else {
            return true;
        }
    }
}