Objectives

_ Creating and compiling a simple program

_ Designing classes

_ Use of recursive algorithms

_ Learn basic combinatorics  

Task: string permutation

Write a recursive method to print all permutations of a string. For example, for a string "abc", the output would be:

abc

acb

bac

bca

cab

cba

Write a program to test your method.

The program should prompt the user to enter a string. Then the program should display all the possible permutations. You can assume that all characters in the input string are different.

Hint:

Declare the following two helping methods:

void displayPermutations(String text)

and

void displayPermutations(String done, String todo)

The first method invokes displayPermutations( "", text ). The second method uses a loop to move a character from todo to done and recursively invokes it with a new todo and done. The base case is when todo string is empty. In this case the method prints done

Example

Note:

user entered values are shown in bold.  

Example 1

This program prints out all permutations of a string.

Enter a string? dome

There are the following permutations:

dome

doem

dmoe

dmeo

deom

demo

odme

odem

omde

omed

oedm

oemd

mdoe

mdeo

mode

moed

medo

meod

edom

edmo

eodm

eomd

emdo

emod

Example 2

This program prints out all permutations of a string.

Enter a string? one

There are the following permutations:

one

oen

noe

neo

eon

eno  

Important Notes

1. For the purposes of this assignment, you can assume that the user will only enter  valid values.

2. In your program you must define and make use of at least the two methods:

a. displayPermutations(String text) and

b. displayPermutations(String done, String todo) as described above.

3. Test your program. In particular, be sure to test it on turing before you submit it.