permutations With First Element - Java Data Structure

Java examples for Data Structure:Algorithm

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Description

permutations With First Element

Demo Code


//package com.java2s;
import java.util.ArrayList;
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);/*from w w  w  . j av  a 2s .co  m*/
        List<List<T>> perms = permutations(list);
        for (List<T> l : perms) {
            l.add(listings.get(0));
        }
        list = new ArrayList<T>();
        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);

        for (List<T> l : perms2) {
            List<T> newList = new ArrayList<T>(l);
            newList.add(listings.get(0));
            perms.add(newList);
        }

        // System.out.println("permutations return list of size " +
        // perms.size());
        return perms;
    }
}

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