Bear And Steady Gene algorithm - two pointer solution, linear solution

BearAndSteadyGene_4th.cs

using System;
using System.Collections.Generic;
using System.IO;

class Solution
{
    /*
     * Julia write the C# code, 
     * code source: Java code
     * https://gist.github.com/jianminchen/d52ced38de0bffa2d9e8
     */
    static void Main(String[] args)
    {
        int N = Int32.Parse(Console.ReadLine());

        string line = Console.ReadLine();

        Console.WriteLine(getMinimumSubstring(N, line));
    }

    public static int getMinimumSubstring(int n, string s)
    {
        int mx = n / 4;

		int[] cnt = new int[256];

		foreach (char c in s) {
			cnt[c]++;
		}

		string t = "ACGT";
		bool ok = true;
		foreach (char c in t) {
			if (cnt[c] > mx) {
				ok = false;
			}
		}

		if (ok) 			
			return 0;		

		int r = 0;

		int ans = n;
		for (int l= 0; l < n; l++) {
			while (cnt['A'] > mx || cnt['C'] > mx || cnt['T'] > mx || cnt['G'] > mx) {
				if (r == n) {					
					return ans;
				}

				--cnt[s[r]];
				++r;
			}
			ans = Math.Min(ans, r - l);
			++cnt[s[l]];
		}

        return ans; 
    }   
}