八皇后问题

import java.util.*;

/**
 * 八皇后问题
 * @author oracs
 * @date 2008-06-04
 */
public class EightQueens {
    private static final int N = 8;
    private int[] queen = new int[N];
    private int count = 0;

    // constructor
    public EightQueens() {
        for (int i = 0; i < N; i++) {
            queen[i] = -1;
        }
    }
    
    /**
     * @param args
     */
    public static void main(String[] args) {
        EightQueens q = new EightQueens();
        q.findQueen(0);
    }
    
    public boolean isQueenAtChest(int i, int j) {
        boolean result = false;
        
        if (j == 0) {  // init queen array
            queen[0] = i;
            return true;
        }
        else {
            // loop queen array, find whether the next queen is in the right positon
            for (int queen_col = 0; queen_col < queen.length; queen_col++) {
                int queen_row = queen[queen_col];
                if ( queen_row != -1) {
                    if ( (i == queen_row) // 同一横线
                       ||((i >= queen_row) && (i == j + (queen_row - queen_col))) // 主对角线
                       ||((i < queen_row) && (i ==  (queen_row + queen_col) - j))) { // 斜对角线
                        
                        return false;
                    }
                }
                
                if (queen_col == queen.length - 1) {
                    queen[j] = i;
                    return true;
                }
            }
        }
        
        return result;
    }
    
    public void findQueen(int col) {
        if (col < N) {
            // loop every row in col
            for (int row = 0; row < N; row++) {
                if (isQueenAtChest(row, col)) {
                    // recursion
                    findQueen(col+1);
                }
            }
            
            // print the queen
            if (col == N - 1) {
                printQueen();
            }
            
            // clear queen
            clearQueen(col);
        }
    }
    
    // print queen
    public void printQueen() {
        for (int i = 0; i < queen.length; i++) {
            if (queen[i] == -1) return;
        }
        
        System.out.println(++count +":" + Arrays.toString(queen));
    }
    
    // initial -1 to queen 's back position
    public void clearQueen(int col) {
        for (int i = col; i < N; i++) {
            queen[i] = -1;
        }
    }

}