C# || How To Generate Unique Permutations From Array With Duplicate Values Using C#

The following is a module with functions which demonstrates how to generate unique permutations from an array with duplicate values using C#.

1. Permute Unique – Problem Statement

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

Example 1:

``` Input: nums = [1,1,2] Output: [[1,1,2], [1,2,1], [2,1,1]] ```

Example 2:

``` Input: nums = [1,2,3] Output: [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]] ```

2. Permute Unique – Solution

The following is a solution which demonstrates how to generate unique permutations from an array with duplicate values.

``` 2. Permute Unique - Solution C# // ============================================================================ // Author: Kenneth Perkins // Date: Oct 27, 2021 // Taken From: http://programmingnotes.org/ // File: Solution.cs // Description: Demonstrates how to generate unique permutations // ============================================================================ public class Solution { private List<IList<int>> result = new List<IList<int>>(); public IList<IList<int>> PermuteUnique(int[] nums) { // Sort Array Array.Sort(nums); Generate(nums, new List<int>(), new bool[nums.Length]); return result; } private void Generate(int[] nums, List<int> combination, bool[] visited) { if (combination.Count == nums.Length) { result.Add(new List<int>(combination)); return; } for (int index = 0; index < nums.Length; ++index) { // Check to see if this number has been visited if (visited[index]) { continue; } else if (index > 0 && nums[index] == nums[index - 1] && !visited[index - 1]) { continue; } // Set that this index has been visited visited[index] = true; // Add this number to the combination combination.Add(nums[index]); // Keep generating permutations Generate(nums, combination, visited); // Unset that this index has been visited visited[index] = false; // Remove last item as its already been explored combination.RemoveAt(combination.Count - 1); } } }// http://programmingnotes.org/ 123456789101112131415161718192021222324252627282930313233343536373839404142434445464748 // ============================================================================//    Author: Kenneth Perkins//    Date:   Oct 27, 2021//    Taken From: http://programmingnotes.org///    File:  Solution.cs//    Description: Demonstrates how to generate unique permutations// ============================================================================public class Solution {    private List<IList<int>> result = new List<IList<int>>();     public IList<IList<int>> PermuteUnique(int[] nums) {        // Sort Array        Array.Sort(nums);        Generate(nums, new List<int>(), new bool[nums.Length]);        return result;    }     private void Generate(int[] nums, List<int> combination, bool[] visited) {        if (combination.Count == nums.Length) {            result.Add(new List<int>(combination));            return;        }         for (int index = 0; index < nums.Length; ++index) {            // Check to see if this number has been visited            if (visited[index]) {                continue;            } else if (index > 0 && nums[index] == nums[index - 1] && !visited[index - 1]) {                continue;            }             // Set that this index has been visited            visited[index] = true;             // Add this number to the combination            combination.Add(nums[index]);             // Keep generating permutations            Generate(nums, combination, visited);             // Unset that this index has been visited            visited[index] = false;             // Remove last item as its already been explored            combination.RemoveAt(combination.Count - 1);        }    }}// http://programmingnotes.org/ ```

QUICK NOTES:
The highlighted lines are sections of interest to look out for.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

Once compiled, you should get this as your output for the example cases:

``` [[1,1,2],[1,2,1],[2,1,1]] [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]] ```