A function to compute all the possible permutations of a given string.

PermuteString.java

import java.util.List;
import java.util.ArrayList;
import java.time.Instant;
import java.time.Duration;

class PermuteString {
	
	static List<String> permutations = new ArrayList<>();

	private static void permuteHelper(String availableChars, String permutationSoFar) {
		if(availableChars.length() == 0){
			permutations.add(permutationSoFar);
			return;
		}

		for(int i = 0; i < availableChars.length(); i++) {
			char currentChar = availableChars.charAt(i);

			String start = availableChars.substring(0, i);
			String end = availableChars.substring(i+1);

			permuteHelper(start+end, permutationSoFar + currentChar);
		}
	}

	private static int fact(int n) {
		if(n == 0 || n == 1)
			return 1;
		else return n * fact(n-1);
	}

	private static void permute(String str){
		permuteHelper(str, "");
	}

	public static void main(String[] args) {
		System.out.println("There will be " + fact(args[0].length()) + " permutations.");
		Instant before = Instant.now();
		permute(args[0]);
		Instant after = Instant.now();
		Duration time = Duration.between(before, after);
		System.out.println(time.getSeconds() + " seconds have elapsed.");
		permutations.forEach(System.out::println);
	}
}