Java排列组合问题

import java.util.Arrays;
import java.util.Stack;

public class Main {
    private static final Stack stack = new Stack<>();
    public static void main(String[] args) {
        int[] nums = {1, 2, 3, 4};
        System.out.println("排列：");
        filter(nums, 4, 0);    // 排列
        stack.clear();
        System.out.println("组合：");
        filter(nums, 3, 0, 0);
    }

    
    static void filter(int[] nums, int target, int count) {
        if (count == target) {
            System.out.println(stack);
            return;
        }
        for (int num : nums) {
            if (!stack.contains(num)) {
                stack.add(num);
                filter(nums, target, count + 1);
                stack.pop();
            }
        }
    }

    
    static void filter(int[] nums, int target, int has, int index) {
        if (has == target) {
            System.out.println(stack);
            return;
        }
        for (int i = index; i < nums.length; i++) {
            if (!stack.contains(nums[i])) {
                stack.add(nums[i]);
                filter(nums, target, has + 1, i);
                stack.pop();
            }
        }
    }
}