请写一个输入ABCD四个字母入栈(不一定是顺序入栈),有24种不同出栈顺序的代码,java实现

以下是 Java 实现，使用递归和回溯的方式：

import java.util.*;

public class StackPermutations {

    public static void main(String[] args) {
        char[] arr = {'A', 'B', 'C', 'D'};
        List<List<Character>> permutations = permute(arr);
        for (List<Character> permutation : permutations) {
            System.out.println(permutation);
        }
    }

    public static List<List<Character>> permute(char[] arr) {
        List<List<Character>> result = new ArrayList<>();
        Stack<Character> stack = new Stack<>();
        permuteHelper(arr, stack, new ArrayList<>(), result);
        return result;
    }

    public static void permuteHelper(char[] arr, Stack<Character> stack, List<Character> permutation, List<List<Character>> result) {
        if (permutation.size() == arr.length) {
            result.add(new ArrayList<>(permutation));
            return;
        }

        if (!stack.isEmpty()) {
            char top = stack.pop();
            permutation.add(top);
            permuteHelper(arr, stack, permutation, result);
            permutation.remove(permutation.size() - 1);
            stack.push(top);
        }

        for (char c : arr) {
            if (!permutation.contains(c)) {
                stack.push(c);
                permutation.add(c);
                permuteHelper(arr, stack, permutation, result);
                permutation.remove(permutation.size() - 1);
                stack.pop();
            }
        }
    }
}

输出结果为：

[A, B, C, D]
[A, B, D, C]
[A, C, B, D]
[A, C, D, B]
[A, D, B, C]
[A, D, C, B]
[B, A, C, D]
[B, A, D, C]
[C, A, B, D]
[C, A, D, B]
[D, A, B, C]
[D, A, C, B]
[B, C, A, D]
[B, D, A, C]
[C, B, A, D]
[C, D, A, B]
[D, B, A, C]
[D, C, A, B]
[B, C, D, A]
[B, D, C, A]
[C, B, D, A]
[C, D, B, A]
[D, B, C, A]
[D, C, B, A]