回溯法求N皇后问题

使用java语言、用回溯法实现N皇后问题，皇后的个数和最后的输出结果都保存在文件中，其中皇后的个数保存在QueensCount.txt文件中，输出结果保存在QueensResult.txt文件中。目录结构如下：

运行结果如下：

源码：

package gavin2;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.OutputStreamWriter;

/**
 * 回溯法--N皇后问题
 * @author Gavin
 *
 */
public class Queen {

	static int sum; // 解数量
	private static String QueensCount = "QueensCount.txt";// 皇后数量文件
	private static String QueensResult = "QueensResult.txt";// 结果文件

	public static void main(String[] args) {
		int n = fileRead(QueensCount);
		File file = null;
		try {
			// 将结果文件清空
			file = new File(QueensResult);
			if (!file.exists()) {
				try {
					file.createNewFile();
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			FileWriter fileWriter = new FileWriter(file, false);
			fileWriter.write("");
			fileWriter.close();
		} catch (IOException e) {
			e.printStackTrace();
		}
		int[] x = new int[n + 1];
		long timeBefore = System.currentTimeMillis();
		NQueens(n, x);
		long timeAfter = System.currentTimeMillis();
		try {
			BufferedWriter bf = new BufferedWriter(new OutputStreamWriter(
					new FileOutputStream(file, true)));
			bf.write("如上所示，当皇后数量为"+n+"时，总共有"+sum+"种结果");
			bf.newLine();
			bf.write("完成算法总共用时："+(timeAfter-timeBefore)/1000+"s");
			bf.flush();
			bf.close();
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
		System.out.println(sum);
	}

	/**
	 * 判断位置是否合法
	 * 
	 * @param i
	 * @return
	 */
	public static boolean place(int x[], int i, int n) {
		if (i > n) {
			return false;
		}
		// x[]是列，i，j是行
		for (int j = 1; j < i; j++) {
			if (x[i] == x[j])
				return false;

			if ((Math.abs(j - i) == Math.abs(x[i] - x[j])))
				return false;
		}
		return true;
	}

	/**
	 * 回溯法解N皇后问题
	 * 
	 * @param n
	 * @param x
	 */
	private static void NQueens(int n, int x[]) {
		int k = 1;// 当前行
		x[1] = 0;// 当前列
		while (k > 0) {// 对所有的行执行以下语句
			x[k] = x[k] + 1;// 移动到下一列
			while (!place(x, k, n) && x[k] <= n) {
				x[k] = x[k] + 1;// 找到位置
			}
			if (x[k] <= n) {
				if (k == n) {
					fileWrite(QueensResult, x);
					sum++;
				} else {
					k = k + 1;
					x[k] = 0;
				}
			} else {
				k = k - 1;
			}
		}
	}

	/**
	 * 读文件
	 * 
	 * @param fileName
	 * @return
	 */
	public static int fileRead(String fileName) {
		int count = 0;
		try {
			BufferedReader bf = new BufferedReader(new FileReader(fileName));
			String str = bf.readLine();
			bf.close();
			count = Integer.parseInt(str);
			return count;
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
		return count;
	}

	/**
	 * 写文件
	 * 
	 * @param fileName
	 * @param a
	 */
	public static void fileWrite(String fileName, int a[]) {
		File file = new File(fileName);
		BufferedWriter bw;
		String str = "";
		String s = "";
		try {
			bw = new BufferedWriter(new OutputStreamWriter(
					new FileOutputStream(file, true)));
			for (int i = 1; i < a.length; i++) {
				str = "x[" + i + "]=" + a[i] + " ";
				s = s + str;
			}
			try {
				System.out.println(s);
				bw.append(s);
				bw.newLine();
				bw.flush();
				bw.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		} catch (IOException e1) {
			e1.printStackTrace();
		}

	}
}