cs: add «1930. Unique Length-3 Palindromic Subsequences»

URL:	https://leetcode.com/problems/unique-length-3-palindromic-subsequences/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/unique-length-3-palindromic-subsequences.cs

@@ -0,0 +1,31 @@
+public class Solution {
+    private static readonly int NOT_FOUND = int.MaxValue;
+
+    public int CountPalindromicSubsequence(string s) {
+        var (first, last) = (new int[26], new int[26]);
+        Array.Fill(first, NOT_FOUND);
+
+        // Initialize the indices
+        for (var i = 0; i < s.Length; ++i) {
+            var idx = s[i] - 'a';
+            first[idx] = Math.Min(first[idx], i);
+            last[idx] = i;
+        }
+
+        var count = 0;
+        for (var c = 0; c < 26; ++c) {
+            if (first[c] == NOT_FOUND) {
+                continue;
+            }
+
+            var between = new HashSet<char>();
+            for (int i = first[c] + 1; i < last[c]; ++i) {
+                between.Add(s[i]);
+            }
+
+            count += between.Count;
+        }
+
+        return count;
+    }
+}