Algo. Buddy Strings

https://leetcode.com/problems/buddy-strings/

/* main idea is to search unmatched symbols, and it shoeld be only 2 of them to swap */

public class Solution {
        public bool BuddyStrings(string s, string goal)
        {
            if (s.Length != goal.Length)
                return false;

            if (s.Equals(goal))
            {
                if (IsDublicateSymbol(s))
                    return true;
                return false;
            }

            var unmatched = 0;
            var sbSource = new StringBuilder();
            var sbGoal = new StringBuilder();
            for (int i = 0; i < s.Length; i++)
            {
                if (s[i] != goal[i])
                {
                    if (unmatched > 2)
                        return false;

                    unmatched++;
                    sbSource.Append(s[i]);
                    sbGoal.Append(goal[i]);
                }
            }

            if (unmatched == 2)
            {
                if (ReverseString(sbSource.ToString()) == sbGoal.ToString())
                    return true;
            }
            else return false;
            
            return false;
        }

        bool IsDublicateSymbol(string s)
        {
            var hashSet = new HashSet<char>();
            foreach (var c in s)
            {
                if (hashSet.Add(c) == false)
                    return true;
            }

            return false;
        }  

        string ReverseString(string s)
        {
            var ca = s.ToCharArray();
            Array.Reverse(ca);
            return new string(ca);
        }
}