/**
 * n queens problem
 *
 * */
public class EightQueen {
    private int QUEEN_COUNT = 0;//represent the queen count
    private int[][] queenCount;//chess box matrix
    private int resultCount = 0;//solution number
    private int[] queenPlace;//mark the queen placed position
    /**
     * construct a EightQueen with a argument represent the number of queen
     * initial a empty chess box
     * 0 represent empty
     * 1 represent the area has been taken
     * recurse to call the putQueen method
     * the queenPlace array to mark the queen's taken area which uses to print
     *
     * */
    public EightQueen(int n) {
        this.QUEEN_COUNT = n;
        this.resultCount = 0;
        this.queenCount = new int[QUEEN_COUNT][QUEEN_COUNT];
        queenPlace = new int[QUEEN_COUNT];
        putQueen(0);
    }
    /**
     * implement the putQueen function to recursion
     * use column index in outer loop and row index in inner loop with step increase
     * */
    private void putQueen(int row) {
        for (int column = 0; column < QUEEN_COUNT; column++) {//loop for QUEEN_COUNT times
            if (queenCount[row][column] == 0) {//judge the condition
                /**
                 * each row has one queen
                 * mark the column and diagonal back diagonal(row has been scatter)
                 *
                 * */
                for (int nextRow = row + 1; nextRow < QUEEN_COUNT; nextRow++) {
                    queenCount[nextRow][column]++;
                    if (column - nextRow + row >= 0) {
                        queenCount[nextRow][column - nextRow + row]++;
                    }
                    if (column + nextRow - row < QUEEN_COUNT) {
                        queenCount[nextRow][column + nextRow - row]++;
                    }
                }
                queenPlace[row] = column;//place the queen with only column information
                if (row == QUEEN_COUNT - 1) printQueen(++resultCount);//recursion has completed
                else putQueen(row + 1);//recurse to call the putQueen function
                /**
                 *
                 * unmarked the column and diagonal back diagonal(row has been scatter)
                 *
                 * */
                for (int rows = row + 1; rows < QUEEN_COUNT; rows++) {
                    queenCount[rows][column]--;
                    if (column - rows + row >= 0) {
                        queenCount[rows][column - rows + row]--;
                    }
                    if (column + rows - row < QUEEN_COUNT) {
                        queenCount[rows][column + rows - row]--;
                    }
                }
            }
        }
        if (row == 0) System.out.println(QUEEN_COUNT + " queens has totally " + resultCount + "result.");
    }

    private void printQueen(int size)
    {
        System.out.println("********** "+size+" **********\n");
        for (int i = 0; i < QUEEN_COUNT; i++) {
            for (int j = 0; j < QUEEN_COUNT; j++) {
                System.out.print(queenPlace[i] == j ? " # " : " - ");
            }
            System.out.println();
        }
    }
}