diff --git a/src/input/21_test1 b/src/input/21_test1
new file mode 100644
index 0000000..9bee783
--- /dev/null
+++ b/src/input/21_test1
@@ -0,0 +1 @@
+list1 = [1,2,4], list2 = [1,3,4]
\ No newline at end of file
diff --git a/src/input/21_test2 b/src/input/21_test2
new file mode 100644
index 0000000..6dbe363
--- /dev/null
+++ b/src/input/21_test2
@@ -0,0 +1 @@
+list1 = [], list2 = []
\ No newline at end of file
diff --git a/src/input/21_test3 b/src/input/21_test3
new file mode 100644
index 0000000..c0a2e69
--- /dev/null
+++ b/src/input/21_test3
@@ -0,0 +1 @@
+list1 = [], list2 = [0]
\ No newline at end of file
diff --git a/src/main.rs b/src/main.rs
index 71e74e5..f146ee4 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -9,6 +9,7 @@ use solutions::*;
 async fn main() -> Result<(), Box<dyn Error>> {
     let problems: Vec<Box<dyn Solution + Sync>> = vec![
         Box::new(valid_parentheses::ValidParentheses {}),
+        Box::new(merge_sorted_lists::MergeSortedLists {}),
         Box::new(xx::XX {}),
     ];
 
diff --git a/src/solutions/merge_sorted_lists.rs b/src/solutions/merge_sorted_lists.rs
new file mode 100644
index 0000000..44aa324
--- /dev/null
+++ b/src/solutions/merge_sorted_lists.rs
@@ -0,0 +1,112 @@
+use super::Solution;
+use crate::utils::linked_list::ListNode;
+
+pub struct MergeSortedLists {}
+
+impl Solution for MergeSortedLists {
+    fn solution(
+        &self,
+        input: &mut Vec<String>,
+    ) -> Result<Box<dyn std::fmt::Display + Sync>, Box<dyn std::error::Error>> {
+        let lists = parse_input(input.first().unwrap());
+        Ok(Box::new(self.merge_two_lists(lists.0, lists.1)))
+    }
+
+    fn get_id(&self) -> u32 {
+        21
+    }
+}
+
+impl MergeSortedLists {
+    pub fn merge_two_lists(
+        &self,
+        list1: Option<Box<ListNode>>,
+        list2: Option<Box<ListNode>>,
+    ) -> Option<Box<ListNode>> {
+        match (list1, list2) {
+            (Some(l1), None) => Some(l1),
+            (None, Some(l2)) => Some(l2),
+            (None, None)     => None,
+            (Some(l1), Some(l2)) => match l1.val <= l2.val {
+                true  => Some(Box::new(ListNode {
+                    val: l1.val,
+                    next: self.merge_two_lists(l1.next, Some(l2)),
+                })),
+                false => Some(Box::new(ListNode {
+                    val: l2.val,
+                    next: self.merge_two_lists(Some(l1), l2.next),
+                })),
+            },
+        }
+    }
+}
+
+fn parse_input(input: &str) -> (Option<Box<ListNode>>, Option<Box<ListNode>>) {
+    let parts: Vec<&str> = input.split(",").collect();
+    let list1_str = parts[0].split('=').nth(1).unwrap().trim();
+    let list2_str = parts[1].split('=').nth(1).unwrap().trim();
+
+    let list1 = ListNode::from_vec(ListNode::parse_list(list1_str));
+    let list2 = ListNode::from_vec(ListNode::parse_list(list2_str));
+
+    (list1, list2)
+}
+
+
+/// Test from puzzle input
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::*;
+
+    #[test]
+    fn test1() {
+        let solution = merge_sorted_lists::MergeSortedLists {};
+
+        //Complete the Challenge
+        let answer = solution
+            .solution(
+                utils::get_input(solution.get_id(), utils::InputType::Test1)
+                    .unwrap()
+                    .as_mut(),
+            )
+            .unwrap()
+            .to_string();
+
+        assert_eq!(answer, "[1,1,2,3,4,4]");
+    }
+
+    #[test]
+    fn test2() {
+        let solution = merge_sorted_lists::MergeSortedLists {};
+
+        //Complete the Challenge
+        let answer = solution
+            .solution(
+                utils::get_input(solution.get_id(), utils::InputType::Test2)
+                    .unwrap()
+                    .as_mut(),
+            )
+            .unwrap()
+            .to_string();
+
+        assert_eq!(answer, "[]");
+    }
+
+    #[test]
+    fn test3() {
+        let solution = merge_sorted_lists::MergeSortedLists {};
+
+        //Complete the Challenge
+        let answer = solution
+            .solution(
+                utils::get_input(solution.get_id(), utils::InputType::Test3)
+                    .unwrap()
+                    .as_mut(),
+            )
+            .unwrap()
+            .to_string();
+
+        assert_eq!(answer, "[0]");
+    }
+}
diff --git a/src/solutions/mod.rs b/src/solutions/mod.rs
index 07a3d1c..61f2c9f 100644
--- a/src/solutions/mod.rs
+++ b/src/solutions/mod.rs
@@ -2,6 +2,7 @@ use crate::utils::{self, get_input};
 use std::{error::Error, fmt::Display, time::SystemTime};
 
 pub mod valid_parentheses;
+pub mod merge_sorted_lists;
 pub mod xx;
 
 pub trait Solution {
diff --git a/src/utils/linked_list.rs b/src/utils/linked_list.rs
new file mode 100644
index 0000000..b87f502
--- /dev/null
+++ b/src/utils/linked_list.rs
@@ -0,0 +1,33 @@
+// Definition for singly-linked list.
+#[derive(PartialEq, Eq, Clone, Debug)]
+pub struct ListNode {
+    pub val: i32,
+    pub next: Option<Box<ListNode>>
+}
+
+impl ListNode {
+    #[inline]
+    pub fn new(val: i32) -> Self {
+        ListNode {
+            next: None,
+            val
+        }
+    }
+
+    pub fn from_vec(vec: Vec<i32>) -> Option<Box<Self>> {
+        let mut current = None;
+        for &v in vec.iter().rev() {
+            let mut new_node = Box::new(ListNode::new(v));
+            new_node.next = current;
+            current = Some(new_node);
+        }
+        current
+    }
+
+    pub fn parse_list(s: &str) -> Vec<i32> {
+        s.trim_matches(|c| c == '[' || c == ']')
+        .split(',')
+        .filter_map(|s| s.trim().parse().ok())
+        .collect()
+    }
+}
diff --git a/src/utils.rs b/src/utils/mod.rs
similarity index 98%
rename from src/utils.rs
rename to src/utils/mod.rs
index 12224ca..4491813 100644
--- a/src/utils.rs
+++ b/src/utils/mod.rs
@@ -1,3 +1,5 @@
+pub mod linked_list;
+
 use std::{
     error::Error,
     fs::File,