Find all permutations of a string. Find all permutations without duplicates.

StringPermutations.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class StringPermutations {

	public List<String> findPermutations(String input) {
		List<Character> charList = new ArrayList<>(input.length());
		for (int i = 0; i < input.length(); i++) {
			charList.add(input.charAt(i));
		}
		
		return findPermutations(new StringBuilder(), charList);
	}

	public List<String> findPermutationsSkipDuplicates(String input) {
		List<Character> charList = new ArrayList<>(input.length());
		for (int i = 0; i < input.length(); i++) {
			charList.add(input.charAt(i));
		}
		
		return findPermutationsSkipDuplicates(new StringBuilder(), charList);
	}
	
	private List<String> findPermutations(StringBuilder prefix, List<Character> remainingElements) {
		List<String> results = new ArrayList<>();
		
		if (remainingElements.size() == 0) {
			return Arrays.asList(prefix.toString());
		}
		
		for (int i = 0; i < remainingElements.size(); i++) {
			List<Character> newRemainingElements = new ArrayList<>(remainingElements);
			Character c = newRemainingElements.remove(i);
			results.addAll(findPermutations(new StringBuilder(prefix.toString()).append(c), newRemainingElements));
		}
		
		return results;
	}
	
	private List<String> findPermutationsSkipDuplicates(StringBuilder prefix, List<Character> remainingElements) {
		List<String> results = new ArrayList<>();
		
		if (remainingElements.size() == 0) {
			return Arrays.asList(prefix.toString());
		}
		
		Set<Character> charsUsed = new HashSet<>();
		
		for (int i = 0; i < remainingElements.size(); i++) {
			if (charsUsed.contains(remainingElements.get(i))) {
				continue;
			}
			List<Character> newRemainingElements = new ArrayList<>(remainingElements);
			Character c = newRemainingElements.remove(i);
			results.addAll(findPermutationsSkipDuplicates(new StringBuilder(prefix.toString()).append(c), newRemainingElements));
			charsUsed.add(c);
		}
		
		return results;
	}
	
	public static void main(String args[]) {
		StringPermutations sp = new StringPermutations();

		for (String permutation : sp.findPermutations("abcdef")) {
			System.out.println(permutation);
		}
		
		System.out.println("==========skip duplicates==========");
		for (String permutation : sp.findPermutationsSkipDuplicates("aabbcc")) {
			System.out.println(permutation);
		}
	}
	
}