diff --git a/5. Longest Palindromic Substring/Program.cs b/5. Longest Palindromic Substring/Program.cs
index 1bc52a6..e7ca51f 100644
--- a/5. Longest Palindromic Substring/Program.cs	
+++ b/5. Longest Palindromic Substring/Program.cs	
@@ -1 +1,18 @@
-Console.WriteLine("Hello, World!");
+var sol = new Solution();
+
+var cases = new (string input, string expected)[]
+{
+	("babad", "bab"),
+	("cbbd", "bb"),
+	("a", "a"),
+	("ac", "a"),
+	("forgeeksskeegfor", "geeksskeeg"),
+	("abba", "abba"),
+	("racecar", "racecar")
+};
+
+foreach (var (input, expected) in cases)
+{
+	var actual = sol.LongestPalindrome(input);
+	Console.WriteLine($"{input} -> {actual} (expected: {expected})");
+}
diff --git a/5. Longest Palindromic Substring/Solutuion.cs b/5. Longest Palindromic Substring/Solutuion.cs
new file mode 100644
index 0000000..c21ac72
--- /dev/null
+++ b/5. Longest Palindromic Substring/Solutuion.cs	
@@ -0,0 +1,56 @@
+using System.Text;
+
+public class Solution
+{
+    public string LongestPalindrome(string s)
+    {
+        if (string.IsNullOrEmpty(s))
+            return "";
+        var str = PrepareString(s);
+        var radii = new int[str.Length];
+        var set = new HashSet<int>(); // Индексы радиусов для обработки
+        var answer = string.Empty;
+
+        for (var i = 0; i < str.Length; i++)
+        {
+            var sym = str[i];
+            var toRemove = new List<int>();
+            foreach (var radiusIdx in set)
+            {
+                var new_radius = radii[radiusIdx] + 1;
+                // Если с новым радиусом выходим за границы
+                if (radiusIdx - new_radius < 0 || radiusIdx + new_radius >= str.Length || str[radiusIdx - new_radius] != sym)
+                {
+                    var len = radii[radiusIdx] * 2 + 1;
+                    if (len > answer.Length)
+                        answer = str.Substring(radiusIdx - radii[radiusIdx], len);
+                    toRemove.Add(radiusIdx);
+                }
+                else
+                    radii[radiusIdx] = new_radius;
+            }
+            foreach (var removeIdx in toRemove)
+                set.Remove(removeIdx);
+
+            set.Add(i);
+        }
+
+        foreach (var radiusIdx in set)
+        {
+            var len = radii[radiusIdx] * 2 + 1;
+            if (len > answer.Length || answer == "#")
+                answer = str.Substring(radiusIdx - radii[radiusIdx], len);
+        }
+
+        return answer.Replace("#", "");
+    }
+
+    public string PrepareString(string s)
+    {
+        var sb = new StringBuilder();
+        sb.Append('#');
+        foreach (var ch in s)
+            sb.Append($"{ch}#");
+        return sb.ToString();
+    }
+}
\ No newline at end of file