Problem

Implement an algorithm to determine if a string has all unique characters.
Example input: "abcdefg"
Example output: True

Solution

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = "abcdefg";
            Console.WriteLine(IsUnique(input));
            Console.ReadLine();
        }

        static bool IsUnique(string input)
        {
            //This solution is optimal because it uses a HashSet to store the characters.
            //HashSets are O(1) for adding and checking for existence.
            //This solution is O(n) because it has to iterate through the entire string.
            HashSet<char> set = new HashSet<char>();
            foreach (char c in input)
            {
                if (set.Contains(c))
                {
                    return false;
                }
                set.Add(c);
            }
            return true;
        }
    }
}