how to find even duplicates character in a string in C# .Net

Question

I have a question of how to remove even duplicates in a string in C#.

example - input string:

acdhqodcqasaf

output:

acdhqosaf

What I really mean is to remove even occurrences of characters.I have written the logic but I have used nested for loops and its efficiency is O(n^2) which is not good efficiency.So I was asked to do in different way searched online still did not get the answer

Answer 1

You could use a dictionary to track the number of occurences and use the % operator:

string input = "acdhqodcqasaf";
var charOccurences = new Dictionary<char, int>();
int removeEvery = 2;
var outputBuilder = new StringBuilder();

foreach (char c in input)
{
    charOccurences.TryGetValue(c, out int charOccurence);
    charOccurence++;
    charOccurences[c] = charOccurence;
    if (charOccurence % removeEvery != 0)
        outputBuilder.Append(c);
}

string output = outputBuilder.ToString();

Answer 2

I'd use a HashSet to keep track of which characters you've seen an odd number of times.

string input = "acdhqodcqasaf";
var oddOccurrences = new HashSet<char>();
var output = new StringBuilder();
foreach (var c in input)
{
    if (!oddOccurrences.Contains(c))
    {
        output.Append(c);
        oddOccurrences.Add(c);
    }
    else
    {
        oddOccurrences.Remove(c);
    }
}
Console.WriteLine(output.ToString());