Queens.java

import java.util.Arrays;

public class Queens {

  public static final int SIZE = 8;
    
  public static void main(String[] args) {
    int grid[][] = new int[SIZE][SIZE];
    int soln[] = new int[SIZE];
    nQ(grid, 0, soln);
  }

  static boolean place(int r, int c, int[][] grid) {
    if (grid[r][c] != 0) {
      return false;
    } else {
      grid[r][c] = 1;
      fill(r, c, grid, true);
      return true;
    }
  }
    
  static void fill(int r, int c, int[][] grid, boolean place) {
    for (int i = r + 1, j = 1; i < grid.length; i++, j++) {
      grid[i][c] += place ? 2 : -2;
      if (c - j >= 0) {
        grid[i][c - j] += place ? 2 : -2;
      }
      if (c + j < grid.length) {
        grid[i][c + j] += place ? 2 : -2;
      }
    }
  }

  static void nQ(int[][] grid, int r, int[] soln) {
    for (int c = 0; c < soln.length; c++) {
      if (place(r, c, grid)) {
        soln[r] = c;
        if (r == soln.length - 1) {
          System.out.println(Arrays.toString(soln));
        } else {
          nQ(grid, r + 1, soln);
        }
        fill(r, c, grid, false);
        grid[r][c] = 0;
      }
    }
  }
}