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| package algorithm;
import java.util.*;
public class NQueensII {
public static int totalNQueens(int n) {
int[] queens = new int[n];
Arrays.fill(queens, -1);
Set<Integer> columns = new HashSet<>();
Set<Integer> diagonals1 = new HashSet<>();
Set<Integer> diagonals2 = new HashSet<>();
// 从第0行开始
return track(queens, n, 0, columns, diagonals1, diagonals2);
}
private static Integer track(int[] queens, int n, int row,
Set<Integer> columns, Set<Integer> diagonals1,
Set<Integer> diagonals2) {
if (row == n) {
return 1;
} else {
int count = 0;
// 每一行都要从第0列开始遍历
for (int i = 0; i < n; i++) {
if (columns.contains(i)) {
continue;
}
int diagonal1 = row - i;
if (diagonals1.contains(diagonal1)) {
continue;
}
int diagonal2 = row + i;
if (diagonals2.contains(diagonal2)) {
continue;
}
queens[row] = i;
columns.add(i);
diagonals1.add(diagonal1);
diagonals2.add(diagonal2);
count += track(queens, n, row + 1, columns, diagonals1, diagonals2);
queens[row] = -1;
columns.remove(i);
diagonals1.remove(diagonal1);
diagonals2.remove(diagonal2);
}
return count;
}
}
public static void main(String[] args) {
System.out.println(totalNQueens(4));
System.out.println(totalNQueens(1));
}
}
|
