回溯法 列出集合S={a1，a2，~，an}的所有全排列

解法一

/**
 * 列出集合S={a1，a2，~，an}的所有全排列
 * @author JH
 *
 */
public class two {

    private static int count=0;
    private static int n;
    private static int[] list;

    public static void pailie(int i) {
        if(i >= n) {
            output();
            return;
        }
        for(int j = i+1; j < n; j++) {
            if(!same(i,j))
                continue;
            swap(i, j);
            pailie(i+1);
            swap(i, j);
        }
        pailie(i+1);
    }

    public static boolean same(int i, int j){  
      for (int k = i; k < j; k++) {  
          if(list[j]==list[k]){  
              return false;  
          }  
      }  
      return true;  
    } 

    public static void swap(int i, int j) {
        list[i] ^= list[j];
        list[j] ^= list[i];
        list[i] ^= list[j];
    }

    public static void output(){
        count++;
        for(int i = 0;i < n;i++) {
            System.out.print(list[i]+" ");
        }
        System.out.println();
    }

    @SuppressWarnings("resource")
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        System.out.print("输入集合内节点数量N");
        n = s.nextInt();
        list = new int[n];
        System.out.print("输入集合内的"+n+"个数");
        for(int i = 0;i < n;i++) {
            list[i] = s.nextInt();
        }
        pailie(0);
        System.out.println("一共"+count+"种全排列");
    }
}

解法二

public class aa {

    private static int k;
    private static int count =0;

    public static void init(int[] list) {
        List<Integer> tmp = new ArrayList<Integer>();
        zuhe(0, tmp, list);
    }

    private static void zuhe(int curIdx, List<Integer>tmp, int[] list) {
        if (curIdx<= list.length&&tmp.size()==k) {
            System.out.println(tmp);
            count++;
        }
        for (int i = curIdx; i<list.length; i++) {
            tmp.add(list[i]);
            zuhe(i + 1, tmp, list);
            tmp.remove(tmp.size() - 1);
        }
    }

    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        System.out.print("输入非负整数N");
        int n = s.nextInt();
        int list[] = new int[n];
        for (int i = 0; i< n; i++) {
            list[i] = i+1;
        }
        System.out.print("输入子集整数个数");
        k = s.nextInt();

        init(list);
        System.out.println(count+"种组合");
    }

}