Minimum queens required to cover all the squares of a chess board

Given the dimension of a chess board (N x M), determine the minimum number of queens required to cover all the squares of the board. A queen can attack any square along its row, column or diagonals. 

Examples:

Input : N = 8, M = 8
Output : 5
Layout : Q X X X X X X X 
         X X Q X X X X X 
         X X X X Q X X X 
         X Q X X X X X X 
         X X X Q X X X X 
         X X X X X X X X 
         X X X X X X X X 
         X X X X X X X X 

Input : N = 3, M = 5
Output : 2
Layout : Q X X X X 
         X X X X X 
         X X X Q X 

This article attempts to solve the problem in a very simple way without much optimization.

• Step 1: Starting from any corner square of the board, find an ‘uncovered’ square (Uncovered square is a square which isn’t attacked by any of the queens already placed). If none found, goto Step 4. 
• Step 2: Place a Queen on this square and increment variable ‘count’ by 1. 
• Step 3: Repeat step 1. 
• Step 4: Now, you’ve got a layout where every square is covered. Therefore, the value of ‘count’ can be the answer. However, you might be able to do better, as there might exist a better layout with lesser number of queens. So, store this ‘count’ as the best value till now and proceed to find a better solution. 
• Step 5: Remove the last queen placed and place it in the next ‘uncovered’ cell. Step 6: Proceed recursively and try out all the possible layouts. Finally, the one with the least number of queens is the answer.

Implementation: Dry run the following code for better understanding. 

Java

// Java program to find minimum number of queens needed
// to cover a given chess board.

public class Backtracking {

    // The chessboard is represented by a 2D array.
    static boolean[][] board;

    // N x M is the dimension of the chess board.
    static int N, M;

    // The minimum number of queens required.
    // Initially, set to MAX_VAL.
    static int minCount = Integer.MAX_VALUE;

    static String layout; // Stores the best layout.

    // Driver code
    public static void main(String[] args)
    {
        N = 8;
        M = 8;
        board = new boolean[N][M];
        placeQueen(0);

        System.out.println(minCount);
        System.out.println("\nLayout: \n" + layout);
    }

    // Finds minimum count of queens needed and places them.
    static void placeQueen(int countSoFar)
    {
        int i, j;

        if (countSoFar >= minCount)

            // We've already obtained a solution with lesser or
            // same number of queens. Hence, no need to proceed.
            return;

        // Checks if there exists any unattacked cells.
        findUnattackedCell : {
        for (i = 0; i < N; ++i)
            for (j = 0; j < M; ++j)
                if (!isAttacked(i, j))

                    // Square (i, j) is unattacked.
                    break findUnattackedCell;

        // All squares all covered. Hence, this
        // is the best solution till now.
        minCount = countSoFar;
        storeLayout();

        return;
        }

        for (i = 0; i < N; ++i)
            for (j = 0; j < M; ++j) {
                if (!isAttacked(i, j)) {

                    // Square (i, j) is unattacked.
                    // Therefore, place a queen here.
                    board[i][j] = true;

                    // Increment 'count' and proceed recursively.
                    placeQueen(countSoFar + 1);

                    // Remove this queen and attempt to
                    // find a better solution.
                    board[i][j] = false;
                }
            }
    }

    // Returns 'true' if the square (row, col) is
    // being attacked by at least one queen.
    static boolean isAttacked(int row, int col)
    {
        int i, j;

        // Check the 'col'th column for any queen.
        for (i = 0; i < N; ++i)
            if (board[i][col])
                return true;

        // Check the 'row'th row for any queen.
        for (j = 0; j < M; ++j)
            if (board[row][j])
                return true;

        // Check the diagonals for any queen.
        for (i = 0; i < Math.min(N, M); ++i)
            if (row - i >= 0 && col - i >= 0 &&
                        board[row - i][col - i])
                return true;
            else if (row - i >= 0 && col + i < M &&
                           board[row - i][col + i])
                return true;
            else if (row + i < N && col - i >= 0 &&
                            board[row + i][col - i])
                return true;
            else if (row + i < N && col + i < M &&
                            board[row + i][col + i])
                return true;

        // This square is unattacked. Hence return 'false'.
        return false;
    }

    // Stores the current layout in 'layout'
    // variable as String.
    static void storeLayout()
    {
        StringBuilder sb = new StringBuilder();
        for (boolean[] row : board) {
            for (boolean cell : row)
                sb.append(cell ? "Q " : "X ");
            sb.append("\n");
        }
        layout = sb.toString();
    }
}

Output:

5

Layout: 
Q X X X X X X X 
X X Q X X X X X 
X X X X Q X X X 
X Q X X X X X X 
X X X Q X X X X 
X X X X X X X X 
X X X X X X X X 
X X X X X X X X