string function calculation - HackerRank - score 0 out of 100 points - only pass one case "aaaaaa"

stringFunctionCalculation1.cs

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

namespace stringFunctionCalculation
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine();

            Console.WriteLine(getMaximumValue(s).ToString()); 
        }

        public static int getMaximumValue(string s)
        {
            if (s == null || s.Length == 0) return 0;

            int n = s.Length;

            int pValue = 1;

            HashSet<string> set = new HashSet<string>();
            for (int i = 1; i < n; i++)  // substring length: i 
            {
                int start = 0;
                int end = i - 1;

                while (start <= n - 1 && end <= n - 1)
                {
                    string s1 = s.Substring(0, i);
                    if (!set.Contains(s1))   // check if set has string 
                    {
                        set.Add(s1);
                        int current = i * getCountSubstring(s1, s);
                        if (current > pValue)
                            pValue = current;
                    }

                    start++;
                    end++;
                }
            }

            return pValue;
        }

        /*
         *  Just go over O(n) - one loop to find the count value 
         */ 
        private static int getCountSubstring(string substring, string s)
        {
            int len1 = substring.Length;
            int len = s.Length;
            if (len1 > len) return 0;

            int start = 0;
            int end = len1 - 1;

            int count = 0; 
            while(start <len  && end < len)
            {
                if(substring.CompareTo(s.Substring(start, len1)) ==0)
                    count++;
                start++;
                end++; 
            }

            return count; 
        }
    }
}