Main.java Source code

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Here is the source code for Main.java

Source

//package com.java2s;
import com.google.common.collect.Lists;
import java.util.Collection;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;

public class Main {
    /** Credit Victor from StackOverflow
     * Combines several collections of elements and create permutations of all of them, taking one element from each
     * collection, and keeping the same order in resultant lists as the one in original list of collections.
     *
     * <ul>Example
     * <li>Input  = { {a,b,c} , {1,2,3,4} }</li>
     * <li>Output = { {a,1} , {a,2} , {a,3} , {a,4} , {b,1} , {b,2} , {b,3} , {b,4} , {c,1} , {c,2} , {c,3} , {c,4} }</li>
     * </ul>
     *
     * @param collections Original list of collections which elements have to be combined.
     * @return Resultant collection of lists with all permutations of original list.
     */
    public static <T> Collection<List<T>> permutations(List<Collection<T>> collections) {
        if (collections == null || collections.isEmpty()) {
            return Collections.emptyList();
        } else {
            Collection<List<T>> res = Lists.newLinkedList();
            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 depth equals number of original collections, final reached, add and return
        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 = Lists.newLinkedList(current);
            copy.add(element);
            permutationsImpl(ori, res, d + 1, copy);
        }
    }
}