Two Characters, by HackerRank

A problem posted by HackerRank on the surface might seem tricky - here is it: https://www.hackerrank.com/challenges/two-characters

String  always consists of two distinct alternating characters. For example, if string 's two distinct characters are xand y, then t could be xyxyx or yxyxy but not xxyy or xyyx.

You can convert some string  to string  by deleting characters from . When you delete a character from , you must delete all occurrences of it in . For example, if  abaacdabd and you delete the character a, then the string becomes bcdbd.

Given , convert it to the longest possible string . Then print the length of string  on a new line; if no string  can be formed from , print  instead.

There are few aspects to notice that will make this problem more manageable:
a) First, the size of the input is small: length of the input string is <= 1000
b) Then the problem is restricted to lowercase alphabetic letters only [a..z], hence 26 letters
c) Finally, the problem is marked as "Easy" by the HackerRank folks

One wrong approach to follow is to attempt to actually delete characters: you'll end up with massive permutations and it will become intractable. One approach to solve the problem is simply brute-force but by trying pairs of potential solutions:

Try pair (a, b)
Now try (a, c)
Now try (a, d)
....
Now try (b, c)
....

And so on. How many such pairs are there? 26*26 = 676. And for each pair, see if it solves the problem by ignoring any other character that is not in the pair. Hence a linear scan in the input string should do it, for a solution in O(n)-time (more precisely an upper bound of 676,000), and O(1)-space.

Code is below - cheers, Marcelo.

using System;

using System.Collections.Generic;
using System.IO;
using System.Linq;
class Solution
{

    static void Main(String[] args)
    {
        int len = Convert.ToInt32(Console.ReadLine());
        string s = Console.ReadLine();

        int longestSolution = 0;
        for (int i = 0; i < 26; i++)
        {
            for (int j = i + 1; j < 26; j++)
            {
                char c1 = (char)((int)'a' + i);
                char c2 = (char)((int)'a' + j);

                int currentChar = -1;
                int countChar = 0;
                for (int z = 0; z < len; z++)
                {
                    if (s[z] == c1)
                    {
                        if (currentChar == 1)
                        {
                            currentChar = -1;
                            break;
                        }
                        currentChar = 1;
                        countChar++;
                    }
                    else if (s[z] == c2)
                    {
                        if (currentChar == 2)
                        {
                            currentChar = -1;
                            break;
                        }
                        currentChar = 2;
                        countChar++;
                    }
                }

                if (currentChar != -1 &&
                    countChar > 1 &&
                    countChar > longestSolution)
                {
                    longestSolution = countChar;
                }
            }
        }
        Console.WriteLine(longestSolution);
    }
}