Leetcode Everyday: 1309. Decrypt String from Alphabet to Integer Mapping. Easy

using System.Text.RegularExpressions;

public class Solution {
    public string FreqAlphabets(string s) {
        var lookup = new Dictionary<string, char>();
        
        var rx = new Regex(@"\d{2}#|\d{1}");
        var alphabet = "12345678910#11#12#13#14#15#16#17#18#19#20#21#22#23#24#25#26#";
        
        var c = 'a';
        foreach (var match in rx.Matches(alphabet).Cast<Match>()) {
            lookup[match.Value] = c++;
        }
        
        var sb = new StringBuilder();        
        foreach (var match in rx.Matches(s).Cast<Match>()) {
            sb.Append(lookup[match.Value]);
        }
        
        return sb.ToString();        
    }
}

Optimized:

public class Solution {
    public string FreqAlphabets(string s) {
        var sb = new StringBuilder();
        for (var i = 0; i < s.Length; ) {
            if (i+2 < s.Length && s[i+2] == '#') {
                sb.Append((char)('a' + (s[i]-'0')*10 + s[i+1]-'0' - 1));                
                i += 3;
            } else {        
                sb.Append((char)('a' + s[i]-'0' - 1));
                ++i;
            }
        }
        return sb.ToString();                
    }
}

Source: https://leetcode.com/problems/decrypt-string-from-alphabet-to-integer-mapping/