N皇后

以下代码效率均很低，只为理解子集树和排列树所用

子集树：

import java.util.Scanner;
public class NQueen {
	static int count = 0, N, b[];

	public static void main(String[] args) {
		N = new Scanner(System.in).nextInt();
		b = new int[N];
		Queen(0);
		System.out.println(count);
	}

	public static boolean place(int k) {
		for (int j = 0; j < k; j++)
			if ((Math.abs(k - j) == Math.abs(b[j] - b[k])) || (b[j] == b[k]))
				return false;
		return true;
	}

	public static void Queen(int n) {
		if (n == N)
			count++;
		else {
			for (int i = 0; i < N; i++) {
				b[n] = i;
				if (place(n))
					Queen(n + 1);
			}
		}
	}
}

排列树：

import java.util.Scanner;
public class EightQueen {
	static int count = 0, N, b[];
	public static void main(String[] args) {
		N = new Scanner(System.in).nextInt();
		b = new int[N];
		for (int i = 0; i < N; i++)
			b[i] = i;
		Queen(0);
		System.out.println(count);
	}

	public static void Queen(int n) {
		if (n == N)
			isOK();
		for (int i = n; i < N; i++) {
			swap(b, i, n);
			Queen(n + 1);
			swap(b, i, n);
		}
	}

	public static boolean isOK() {
		for (int i = 0; i < N; i++)
			for (int j = i + 1; j < N; j++)
				if (Math.abs(i - j) == Math.abs(b[i] - b[j]))
					return false;
		count++;
		return true;
	}
	public static void swap(int a[], int i, int j) {
		int temp = a[i];
		a[i] = a[j];
		a[j] = temp;
	}
}