Java函数式编程中递归式回溯算法的实现技巧

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class EightQueens {

    public static void main(String[] args) {
        int n = 8;
        Stream<int[]> solutions = solveNQueens(n);
        List<int[]> result = solutions.collect(Collectors.toList());
        System.out.println("Total solutions: " + result.size());
        for (int[] solution : result) {
            System.out.println(Arrays.toString(solution));
        }
    }

    public static Stream<int[]> solveNQueens(int n) {
        return Stream.iterate(new int[n], EightQueens::nextBoard)
                .takeWhile(Arrays::nonNull)
                .filter(EightQueens::isSafe);
    }

    private static int[] nextBoard(int[] previous) {
        int[] next = Arrays.copyOf(previous, previous.length);
        int queenCount = (int) Arrays.stream(next).filter(i -> i != 0).count();
        if (queenCount == next.length) {
            return null;
        }
        int nextQueenColumn = queenCount;
        for (int i = 0; i < next.length; i++) {
            if (previous[i] == 0) {
                next[i] = nextQueenColumn + 1;
                return next;
            }
        }
        return null;
    }

    private static boolean isSafe(int[] board) {
        for (int i = 0; i < board.length; i++) {
            for (int j = i + 1; j < board.length; j++) {
                if (board[i] == board[j]) {
                    return false;
                } else if (Math.abs(board[i] - board[j]) == j - i) {
                    return false;
                }
            }
        }
        return true;
    }
}