47. Permutations II

Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:

[1,1,2], [1,2,1], and [2,1,1].

Solution 1 Use set

public List<List<Integer>> permuteUnique(int[] num) {Set<List<Integer>> permutations = new HashSet<List<Integer>>();if (num.length > 0) {permutations.add(Arrays.asList(num[0]));for (int index = 1; index < num.length; index++) {Set<List<Integer>> newPermutations = new HashSet<List<Integer>>();for (List<Integer> list : permutations) {for (int innerIndex = 0; innerIndex <= list.size(); innerIndex++) {List<Integer> newList = new ArrayList(list);newList.add(innerIndex, num[index]);newPermutations.add(newList);}}permutations = newPermutations;}}return new ArrayList<List<Integer>>(permutations);}

Solution 2

public static List<List<Integer>> permuteUnique2(int[] nums) {List<List<Integer>> res = new ArrayList<List<Integer>>();Arrays.sort(nums);LinkedList<Integer> list = new LinkedList<Integer>();for (int num : nums)list.add(num);permute2(list, 0, res);return res;}private static void permute2(LinkedList<Integer> nums, int start, List<List<Integer>> res) {if (start == nums.size() - 1) {res.add(new LinkedList<Integer>(nums));return;}for (int i = start; i < nums.size(); i++) {if (i > start && nums.get(i) == nums.get(i - 1))continue;nums.add(start, nums.get(i));// swapnums.remove(i + 1);permute2(nums, start + 1, res);nums.add(i + 1, nums.get(start));nums.remove(start);}}

Solution 3 Same idea with the last one

/* * The idea is for each recursion level, swap the current element at 1st * index with each element that comes after it (including itself) */// https://leetcode.com/discuss/84305/share-my-java-code-with-detailed-explanantionpublic List<List<Integer>> permuteUnique3(int[] nums) {List<List<Integer>> ans = new ArrayList<>();if (nums == null || nums.length == 0) {return ans;}permute(ans, nums, 0);return ans;}private void permute(List<List<Integer>> ans, int[] nums, int index) {if (index == nums.length - 1) {List<Integer> temp = new ArrayList<>();for (int num : nums) {temp.add(num);}ans.add(temp);return;}Set<Integer> appeared = new HashSet<>();for (int i = index; i < nums.length; ++i) {if (appeared.add(nums[i])) {swap(nums, index, i);permute(ans, nums, index + 1);swap(nums, index, i);}}}private void swap(int[] nums, int i, int j) {int save = nums[i];nums[i] = nums[j];nums[j] = save;}