// CSE 143, Winter 2010, Marty Stepp
// This program uses recursive backtracking to print all permutations
// (all possible arrangements of letters) of a given string.

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) {
        // parameters desired:
        //   1. letters available to choose
        //   2. letters we already have chosen
        permute(s, "");
    }
    
    // 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) {
        // uncomment the following line to see the recursive calls
        // Recursion.println("permute " + s + " " + chosen);
        
        if (s.length() == 0) {
            // base case: all letters have been chosen.  Print the chosen word
            System.out.println(chosen);
        } else {
            // for each possible next letter to try, ...
            for (int i = 0; i < s.length(); i++) {
                // choose
                char c = s.charAt(i);
                s = s.substring(0, i) + s.substring(i + 1);
                
                // explore
                permute(s, chosen + c);
                
                // un-choose
                s = s.substring(0, i) + c + s.substring(i);
            }
        }
    }

    // permute("marty", "")
    //     permute("arty", "m")
    //         permute("rty", "ma")
    //             permute("", "matry")   --> matry
}