Permutations.java

package String;

import java.util.*;

public class Permutations {

    public static List<String> generateUniquePermutations(String str) {
        List<String> result = new ArrayList<>();
        char[] chars = str.toCharArray();

        generateUniquePermutations(chars, 0, chars.length - 1, result);
        return result;
    }

    public static void generateUniquePermutations(char[] chars, int start, int end, List<String> result) {
        if (start == end) {
            result.add(new String(chars));
            return;
        }

        Set<Character> seen = new HashSet<>();
        for (int i = start; i <= end; i++) {
            if (!seen.contains(chars[i])) {
                seen.add(chars[i]);
                swap(chars, i, start);
                generateUniquePermutations(chars, start + 1, end, result);
                swap(chars, i, start);
            }
        }
    }

    public static void swap(char[] chars, int i, int j) {
        char temp = chars[i];
        chars[i] = chars[j];
        chars[j] = temp;
    }

    public List<String> find_permutation(String S) {
        List<String> res = generateUniquePermutations(S);
        Collections.sort(res);
        return res;
    }
}