permutations of a string using iteration

Question

I\'m trying to find permutation of a given string, but I want to use iteration. The recursive solution I found online and I do understand it, but converting it to an iterati

Answer 1

Work queue allows us to create an elegant iterative solution for this problem.

static List permutations(String string) {
    List permutations = new LinkedList<>();
    Deque workQueue = new LinkedList<>(); 

    // We need to permutate the whole string and haven't done anything yet.
    workQueue.add(new WorkUnit(string, ""));

    while (!workQueue.isEmpty()) { // Do we still have any work?
        WorkUnit work = workQueue.poll();

        // Permutate each character.
        for (int i = 0; i < work.todo.length(); i++) {
            String permutation = work.done + work.todo.charAt(i);

            // Did we already build a complete permutation?
            if (permutation.length() == string.length()) {
                permutations.add(permutation);
            } else {

                // Otherwise what characters are left? 
                String stillTodo = work.todo.substring(0, i) + work.todo.substring(i + 1);
                workQueue.add(new WorkUnit(stillTodo, permutation));
            }
        }
    }
    return permutations; 
}

A helper class to hold partial results is very simple.

/**
 * Immutable unit of work
 */
class WorkUnit {
    final String todo;
    final String done;

    WorkUnit(String todo, String done) {
        this.todo = todo;
        this.done = done;
    }
}

You can test the above piece of code by wrapping them in this class.

import java.util.*;

public class AllPermutations {

    public static void main(String... args) {
        String str = args[0];
        System.out.println(permutations(str));
    }

    static List permutations(String string) {
        ...
    }
}

class WorkUnit {
    ...
}

Try it by compiling and running.

$ javac AllPermutations.java; java AllPermutations abcd

The below implementation can also be easily tweaked to return a list of permutations in reverse order by using a LIFO stack of work instead of a FIFO queue.