第一元素置换

//package com.nowjava;

import java.util.ArrayList;/** from nowjava**/

import java.util.List;


public class Main {

    public static <T> List<List<T>> permutationsWithFirstElement(

            List<T> listings) throws Exception {

        List<T> list = new ArrayList<T>(listings);

        list.remove(0);

        List<List<T>> perms = permutations(list);

        for (List<T> l : perms) {

            l.add(listings.get(0));

        }

        list = new ArrayList<T>();/*来 自 n o w j a v a . c o m - 时  代  Java*/

        list.add(listings.get(0));

        perms.add(list);

        return perms;

    }


    public static <T> List<List<T>> permutations(List<T> listings)

            throws Exception {

        /*

         * String debug = "["; for (int i = 0; i < listings.size(); ++i) { debug

         * += listings.get(i).toString(); if (i < (listings.size() - 1)) { debug

         * += ", "; } } debug += "]"; System.out.println("permutations(" + debug

         * + ")");

         */


        List<List<T>> perms = new ArrayList<List<T>>();


        if (listings.size() == 0) {

            // System.out.println("permutations return list of size 0");

            return perms;

        }


        List<T> list = new ArrayList<T>();

        list.add(listings.get(0));

        perms.add(list);


        if (listings.size() == 1) {

            // System.out.println("permutations return list of size " +

            // perms.size());

            return perms;

        }


        List<List<T>> perms2 = permutations(listings.subList(1,

                listings.size()));

        perms.addAll(perms2);