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N Queens

1. You are given a number n, the size of a chess board.

2. You are required to place n number of queens in the n * n cells of board such that no queen can kill another.

Note - Queens kill at distance in all 8 directions

3. Complete the body of printNQueens function - without changing signature - to calculate and print all safe configurations of n-queens. Use sample input and output to get more idea.

Input Format

A number n

Output Format

Safe configurations of queens as suggested in sample output


Constraints

1 <= n <= 10

Sample Input

4

Sample Output

0-1, 1-3, 2-0, 3-2, .

0-2, 1-0, 2-3, 3-1, .

Solution:

import java.io.*;

import java.util.*;

public class Main {

  public static void main(String[] args) throws Exception {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    int n = Integer.parseInt(br.readLine());
    int[][] chess = new int[n][n];
    printNQueens(chess, "", 0);
  }

  public static void printNQueens(int[][] chess, String qsf, int row) {
    if (row == chess.length) {
      System.out.println(qsf + ".");
      return;
    }
    for (int col = 0; col < chess.length; col++) {
      if (chess[row][col] == 0
          && isQueenSafe(chess, row, col) == true) {
        chess[row][col] = 1;
        printNQueens(chess,
                     qsf + row + "-" + col + ", ", row + 1);
        chess[row][col] = 0;
      }
    }
  }


  public static boolean isQueenSafe(int[][] chess,
                                    int row, int col) {
    for (int i = row - 1, j = col - 1;
         i >= 0 && j >= 0; i--, j--) {
      if (chess[i][j] == 1) {
        return false;
      }
    }

    for (int i = row - 1, j = col; i >= 0; i--) {
      if (chess[i][j] == 1) {
        return false;
      }
    }

    for (int i = row - 1, j = col + 1; i >= 0
         && j < chess.length; i--, j++) {
      if (chess[i][j] == 1) {
        return false;
      }
    }

    for (int i = row, j = col - 1; j >= 0; j--) {
      if (chess[i][j] == 1) {
        return false;
      }
    }

    return true;
  }
}
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