以下代碼效率均很低,只爲理解子集樹和排列樹所用
子集樹:
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);
}
}
}
}
排列樹:
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;
}
}