// CSE 143, Winter 2010, Marty Stepp
// This program uses recursive backtracking to print all permutations
// (all possible arrangements of letters) of a given string.
// 
// This file has an alternative implementation of the "subsets" problem
// that uses lists rather than strings / chars to store choices.

import java.util.*;

public class Permutations {
    // turn on the DEBUG flag to see a bunch of helpful println statements
    // to watch the recursion trace
    private static final boolean DEBUG = false;
    
    public static void main(String[] args) {
        permute("MARTY");
        System.out.println();
        permute("JANE");
    }
    
    // Prints all possible permutations of letters of the given string,
    // one per line.  For example, permute("MARTY") prints MARTY, MATRY,
    // ATRMY, YARMT, etc.  Uses backtracking to generate permutations.
    // Precondition: s is not null  (throws NullPointerException if so)
    public static void permute(String s) {
        // represent the choices as a list of 1-letter strings
        List<String> choices = new ArrayList<String>();
        for (int i = 0; i < s.length(); i++) {
            choices.add(s.substring(i, i + 1));
        }
        
        // initially we have chosen zero letters
        List<String> chosen = new ArrayList<String>();
        
        permute(choices, chosen);
    }
    
    // Recursive helper method for choosing letters and backtracking.
    // Additional 'chosen' parameter stores the letters we have picked so far.
    private static void permute(List<String> choices, List<String> chosen) {
        // uncomment the following line to see the recursive calls
        if (DEBUG) Recursion.println("PERMUTE(" + choices + ", " + chosen + ")");
        
        if (choices.size() == 0) {
            // base case: all letters have been chosen.  Print the chosen word
            if (DEBUG) Recursion.println("BASE CASE: " + chosen);
            for (String s : chosen) {
                System.out.print(s);
            }
            System.out.println();
        } else {
            // for each possible choice i:
            //     choose i.
            //     explore the rest.
            //     un-choose i.
            for (int i = 0; i < choices.size(); i++) {    // [M, A, R, T, Y]    []
                // choose
                String myChoice = choices.remove(i);      // "M"
                chosen.add(myChoice);                     // [A, R, T, Y]    [M]
                if (DEBUG) Recursion.println("MAKE A CHOICE: " + myChoice);
                
                // explore the rest
                permute(choices, chosen);
                
                // un-choose
                chosen.remove(chosen.size() - 1);
                choices.add(i, myChoice);
                if (DEBUG) Recursion.println("UN-MAKE A CHOICE: " + myChoice);
            }
        }
    }
}