diff --git a/src/lib.rs b/src/lib.rs
index 7a0f920..8f95e4c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -322,6 +322,7 @@ pub mod problem_0398_random_pick_index;
 pub mod problem_0400_nth_digit;
 pub mod problem_0404_sum_of_left_leaves;
 pub mod problem_0405_convert_a_number_to_hexadecimal;
+pub mod problem_0409_longest_palindrome;
 pub mod problem_0412_fizz_buzz;
 pub mod problem_0413_arithmetic_slices;
 pub mod problem_0415_add_strings;
diff --git a/src/problem_0409_longest_palindrome/iterative.rs b/src/problem_0409_longest_palindrome/iterative.rs
new file mode 100644
index 0000000..fb4716e
--- /dev/null
+++ b/src/problem_0409_longest_palindrome/iterative.rs
@@ -0,0 +1,27 @@
+pub struct Solution;
+
+impl Solution {
+    pub fn longest_palindrome(s: String) -> i32 {
+        let mut chs = [0; 128];
+        s.bytes().for_each(|x| chs[x as usize] += 1);
+
+        let result = chs
+            .into_iter()
+            .fold(0, |acc, count| acc + (count - count % 2));
+        result + i32::from(result as usize != s.len())
+    }
+}
+
+impl super::Solution for Solution {
+    fn longest_palindrome(s: String) -> i32 {
+        Self::longest_palindrome(s)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_solution() {
+        super::super::tests::run::<super::Solution>();
+    }
+}
diff --git a/src/problem_0409_longest_palindrome/mod.rs b/src/problem_0409_longest_palindrome/mod.rs
new file mode 100644
index 0000000..8ec4fc5
--- /dev/null
+++ b/src/problem_0409_longest_palindrome/mod.rs
@@ -0,0 +1,18 @@
+pub mod iterative;
+
+pub trait Solution {
+    fn longest_palindrome(s: String) -> i32;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Solution;
+
+    pub fn run<S: Solution>() {
+        let test_cases = [("abccccdd", 7), ("a", 1), ("bb", 2)];
+
+        for (s, expected) in test_cases {
+            assert_eq!(S::longest_palindrome(s.to_string()), expected);
+        }
+    }
+}