Main.java Source code

Introduction

Here is the source code for Main.java

Source

//package com.java2s;
//License from project: Open Source License 

import java.util.Collection;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;

public class Main {
    /**
     * Generates permutations from a list of collections. Each permutation contains an element
     * from each of the collections. 
     * 
     * @param <T>         the generic type for object in the collections
     * @param collections   the list of collections
     * @return            the collection of permutations
     */
    public static <T> Collection<List<T>> generatePermutations(List<Collection<T>> collections) {
        if (collections == null || collections.isEmpty()) {
            return Collections.emptyList();
        } else {
            Collection<List<T>> res = new LinkedList<List<T>>();
            permutationsImpl(collections, res, 0, new LinkedList<T>());
            return res;
        }
    }

    private static <T> void permutationsImpl(List<Collection<T>> ori, Collection<List<T>> res, int d,
            List<T> current) {
        if (d == ori.size()) {
            res.add(current);
            return;
        }
        // iterate from current collection and copy 'current' element N times, one for each element
        Collection<T> currentCollection = ori.get(d);
        for (T element : currentCollection) {
            List<T> copy = new LinkedList<T>();
            copy.addAll(current);
            copy.add(element);
            permutationsImpl(ori, res, d + 1, copy);
        }
    }
}