CareerCup

public static Main(string[] args)
{
	public VotingSystem votingSystem = VotingSystem.GetInstance();
	while(true)
	{
		// Add candidates
		break;
	}
	while(true)
	{
		// Add people vote.
	}
	var candidate = votingSystem.GetWinner();
	int winnerVoteCount = votingSystem.GetVoteCount(candidate);
}

public class VotingSystem
{
	private Dictionary<Candidate, int> candidatesVoteCount;
	private VotingSystem votingSystem;
	private VotingSystem()
	{
		candidatesVoteCount = GetExistingCandidateInformationFromDB();
	}
	public VotingSystem GetInstance()
	{
		if(votingSystem === null)
		{
			votingSystem =  new VotingSystem();
		}
		return votingSystem;
	}
	public bool AddCandidate(string name, PartyName partyName)
	{
		var candidate = new Candidate(name, partyName);
		if(candidatesVoteCount.ContainsKey(candidate))
		{
			throw new InvalidArgumentException("input candidate is already present");
		}
		candidatesVoteCount.Add(candidate, 0);
		return true;
	}
	public bool Remove(string name, PartyName partyName)
	{
		var candidate = new Candidate(name, partyName);
		if(candidatesVoteCount.ContainsKey(candidate))
		{
			candidatesVoteCount.Remove(candidate);
			return true;
		}
		throw new InvalidArgumentException("Input Candidate is not present in the system");
	}
	public bool AddVote(string name, PartyName partyName)
	{
		var candidate = new Candidate(name, partyName);
		if(candidatesVoteCount.ContainsKey(candidate))
		{
			candidatesVoteCount[candidate]++;
			return true;
		}
		throw new InvalidArgumentException("Input Candidate is not present in the system");
	}
	public Candidate GetWinner()
	{
		candidatesVoteCount.Aggregate((l, r) => l.Value > r.Value ? l : r).Key;
	}
	public int GetVoteCount(string name, PartyName partyName)
	{
		var candidate = new Candidate(name, partyName);
		if(candidatesVoteCount.ContainsKey(candidate))
		{
			return candidatesVoteCount[candidate];
		}
		throw new InvalidArgumentException("Input Candidate is not present in the system");
	}
	public int GetVoteCount(Candidate candidate)
	{
		if(candidatesVoteCount.ContainsKey(candidate))
		{
			return candidatesVoteCount[candidate];
		}
		throw new InvalidArgumentException("Input Candidate is not present in the system");
	}
	public GetTotalVoteCount()
	{
		return candidatesVoteCount.Sum(x => x.Value);
	}
	private Dictionary<Candidate, int> GetExistingCandidateInformationFromDB()
	{
		var map = new Dictionary<Candidate, int>();
		var rowSets = GetExistingCandidates(); // this method will call its internal db and return informations about the candidates
		foreach(Row row in rowSets)
		{
			map.Add(new Candidate(row[0], Enum.Parse(row[1], typeof(PartyName))),Int32.Parse(row[2]));
		}
		return map;
	}
}

public class Candidate
{
	public string name;
	public PartyName partyName;
	public Candidate(string name, PartyName partyName)
	{
		this.name = name;
		this.partyName = partyName;
	}
}

public Enum PartyName
{
	"a" = 1,
	"b" = 2
}

Complexities are
Add Candidate: O(1)
Add Vote: O(1)
Delete Candidate: O(1)
GetWinner : O(N), N is total candidate count // least used
GetCandidateVoteCount : O(1)
GetTotalVoteCount : O(N), can be made O(1) easily. It is scalable, can store data into the database and while we restart the machine, it can read data from the DB and initialize itself. We can very well optimize GetWinner call as well, to return output in O(1) time. To do that, we have to store, the candidate vote count in a LinkedList and in the dictionary, we will store candidate information and corresponding LinkedList node for the vote count. We will also store maxVoteCount variable outside of all these, and when the current candidate vote count become more then maxVoteCount, then we update maxVoteCount and put that node in the first of the LinkedList. By this way, we will be able to answer GetWinner as well in O(1), just that insert and delete will take more time not in terms of complexity but in terms of more computer operations.
Failure conditions are all taken care of.