cs: add «2981. Find Longest Special Substring That Occurs Thrice I»

URL:	https://leetcode.com/problems/find-longest-special-substring-that-occurs-thrice-i/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/find-longest-special-substring-that-occurs-thrice-i.cs

@@ -0,0 +1,26 @@
+public class Solution {
+    public int MaximumLength(string s) {
+        var counter = new Dictionary<(char, int), int>();
+
+        for (var l = 0; l < s.Length; ++l) {
+            var c = s[l];
+
+            var substringLength = 0;
+            for (var r = l; r < s.Length && c == s[r]; ++r) {
+                ++substringLength;
+
+                var key = (c, substringLength);
+                counter[key] = 1 + (counter.ContainsKey(key) ? counter[key] : 0);
+            }
+        }
+
+        return counter.Select(pair => {
+            if (pair.Value < 3) {
+                return null;
+            }
+
+            var (c, length) = pair.Key;
+            return (int?)length;
+        }).Max() ?? -1;
+    }
+}