题解 | #火车进站#

import java.util.*;


public class Main {

    public static List<List<Integer>> getAllPermutations(int[] input) {
        List<List<Integer>> result = new ArrayList<>();
        Stack<Integer> stack = new Stack<>();
        generatePermutations(input, 0, stack, new ArrayList<>(), result);
        return result;
    }

    private static void generatePermutations(int[] input, int index, Stack<Integer> stack, List<Integer> current, List<List<Integer>> result) {
        if (index == input.length && stack.isEmpty()) {
            // 当入栈顺序遍历完成且栈为空时，将当前出栈顺序加入结果集，注意这里是new了一个list。
            result.add(new ArrayList<>(current));
            return;
        }

        // 入栈
        if (index < input.length) {
            stack.push(input[index]);
            generatePermutations(input, index + 1, stack, current, result);
            stack.pop();
        }

        // 出栈
        if (!stack.isEmpty()) {
            int popped = stack.pop();
            current.add(popped);
            generatePermutations(input, index, stack, current, result);
            current.remove(current.size() - 1);
            stack.push(popped);
        }
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int count = sc.nextInt();
        int[] input = new int[count];
        for(int i = 0;i < count;i++){
            input[i] = sc.nextInt();
        }
        
        List<List<Integer>> permutations = getAllPermutations(input);

        permutations.sort((o1,o2) ->{
            for(int i = 0;i < o1.size();i++){
                int result = o1.get(i) - o2.get(i);
                if(result != 0){
                    return result;
                }
            }
            return 0;
        });

        for (List<Integer> permutation : permutations) {
            for(int a:permutation){
                System.out.print(a+ " ");

            }
            System.out.println();
        }
    }
}

感觉写的挺规整的，记录一下。