/*

Problem Description

​

Given an array of positive integers A, one integer appear only once, and all the other integers appear thrice.

Find the integer that appear only once.

*/

using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        List<int> nums = [1, 2, 4, 3, 3, 2, 2, 3, 1, 1];

        int n = nums.Count;

        var isSetBit = (int num, int pos) => ((num >> pos) & 1) == 1;

        int ans = 0;

        for (int bitPos = 0; bitPos < 32; bitPos++)

        {

            int setBitCount = 0;

            nums.ForEach((num) =>

            {

                if (isSetBit(num, bitPos))

                {

                    setBitCount++;

                }

            });

            if (setBitCount % 3 != 0)

                ans |= 1 << bitPos;

        }

​

        Console.WriteLine($"Missing number is {ans}");

        // Anotherway of doing using LINQ

        var singleNumber = nums.GroupBy(x => x)

                               .OrderBy(k => k.Count())

                               .First().Key;

        Console.WriteLine($"Single number is {singleNumber}");

    }

}