// CSE 143, Winter 2011, Marty Stepp                                   
// This program uses recursive backtracking to print all permutations                                        
// (all possible arrangements of letters) of a given string.                                               
                                               
import java.util.*;                                                   
                                     
public class Permutations {                               
    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) {                                   
        permute(s, "");                                        
    }                                               
                                               
    // s: set of letters available to choose                                                   
    // chosen: set of letters I already chose                                     
    // s = "MARTY", chosen = ""                               
    // s = "ARTY" , chosen = "M"                               
    // s = "ARY"  , chosen = "MT"                                       
    // ....                                               
    // s = ""     , chosen = "MTRYA"                                               
                                    
    // Recursive helper method for choosing letters and backtracking.                               
    // Additional 'chosen' parameter stores the letters we have picked so far.                                                  
    private static void permute(String s, String chosen) {                                     
        // System.out.println("permute " + s + ", " + chosen);                                            
        if (s.length() == 0) {                                 
            System.out.println(chosen);  // base case                                                    
        } else {                                         
            for (int i = 0; i < s.length(); i++) {                                               
                // choose                                              
                String chosenLetter = s.substring(i, i + 1);                                                   
                s = s.substring(0, i) + s.substring(i + 1, s.length());        // remove firstLetter from s                                        
                chosen += chosenLetter;                                         
                                                
                // explore                               
                permute(s, chosen);                                 
                                    
                // un-choose                                                      
                chosen = chosen.substring(0, chosen.length() - 1);                                         
                s = s.substring(0, i) + chosenLetter + s.substring(i);                                   
            }                                     
        }
    }

    // an alternative implementation that takes advantage of the fact that
    // strings are immutable to make the "choosing" and "un-choosing" easier
    /*
    private static void permute(String s, String chosen) {
        Recursion.println("permute " + s + ", " + chosen);
        if (s.length() == 0) {
            System.out.println(chosen);  // base case
        } else {
            for (int i = 0; i < s.length(); i++) {
                // choose
                String chosenLetter = s.substring(i, i+1);
                String newS = s.substring(0, i) + s.substring(i+1, s.length());        // remove firstLetter from s
                String newChosen = chosen + chosenLetter;
                
                // explore
                permute(newS, newChosen);
                
                // un-choose
                // nothing to do!
            }
        }
    }
    */
}