JAVA 实现全排列和组合

参考了这篇文章
https://blog.csdn.net/leixingbang1989/article/details/47761171

代码：

import java.util.LinkedList;
import java.util.List;
import java.util.Scanner;

public class Main {

public static void arrange(String s, List<Integer> list) {
    if (list.size() == 0) {
        System.out.println(s);
    } else {
        for (int i = 0; i < list.size(); ++i) {
            List<Integer> temp = new LinkedList<>(list);
            //取出一个数，剩下的数进行全排列
            Integer remove = temp.remove(i);
            arrange(s + remove, temp);
            temp = null;
        }
    }
}

public static void group(String s, List<Integer> list, int n) {
    if (n == 0) {
        System.out.println(s);
    } else {
        for (int i = 0; i < list.size(); ++i) {
            Integer integer = list.get(i);
            int length = s.length();
            //限定前面的数比后面的小，用来去重复，例如 1 2 和2 1 
            if(length != 0 && s.charAt(length - 1) > integer + '0'){
                continue;
            }
            List<Integer> temp = new LinkedList<>(list);
            Integer remove = temp.remove(i);
            group(s + remove, temp, n - 1);
            temp = null;
        }
    }
}

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    List<Integer> list = new LinkedList<>();
    for (int i = 1; i <= n; ++i) {
        list.add(i);
    }
    arrange("", list);
    System.out.println();
    group("", list, 2);
    System.out.println();
}
}

---

2018/3/28补充
不用每次都新建一个list，只需要将之前remove掉的值交换，换到前面即可

public static void swap(List<Integer> list, int i, int j){
    Integer src = list.get(i);
    Integer dst = list.get(j);
    Integer temp = src;
    src = dst;
    dst = temp;
}

public static void arrange2(String s, List<Integer> list, int start) {
    if (start == list.size()) {
        System.out.println(s);
    } else {
        for (int i = start; i < list.size(); ++i) {
            swap(list, start, i);
            arrange2(s + list.get(start), list, start + 1);
            swap(list, start, i);
        }
    }
}

可以参考https://blog.csdn.net/qq_34115899/article/details/79733654

然而，上面这段代码是错误的
错误的
错误的
错误的
因为这样根本没办法交换list里面的2个值
原因请看

解决方法，可以用list的set方法，也可以改成int数组