diff --git a/include/s0021_merge_two_sorted_lists.hpp b/include/s0021_merge_two_sorted_lists.hpp
new file mode 100644
index 0000000..152ca8b
--- /dev/null
+++ b/include/s0021_merge_two_sorted_lists.hpp
@@ -0,0 +1,19 @@
+#ifndef S0021_MERGE_TWO_SORTED_LISTS
+#define S0021_MERGE_TWO_SORTED_LISTS
+
+struct ListNode {
+  int val;
+  ListNode* next;
+  ListNode() : val(0), next(nullptr) {}
+  ListNode(int x) : val(x), next(nullptr) {}
+  ListNode(int x, ListNode* next) : val(x), next(next) {}
+};
+
+#include <limits>
+
+class Solution {
+ public:
+  ListNode* mergeTwoLists(ListNode* list1, ListNode* list2);
+};
+
+#endif
diff --git a/src/s0021_merge_two_sorted_lists.cpp b/src/s0021_merge_two_sorted_lists.cpp
new file mode 100644
index 0000000..73b83bd
--- /dev/null
+++ b/src/s0021_merge_two_sorted_lists.cpp
@@ -0,0 +1,40 @@
+#include "s0021_merge_two_sorted_lists.hpp"
+
+ListNode* Solution::mergeTwoLists(ListNode* list1, ListNode* list2) {
+  if (list1 == nullptr) {
+    return list2;
+  } else if (list2 == nullptr) {
+    return list1;
+  }
+  ListNode dummy = ListNode(-std::numeric_limits<int>::max());
+  if (list1->val > list2->val) {
+    dummy.next = list2;
+    list2 = list2->next;
+  } else {
+    dummy.next = list1;
+    list1 = list1->next;
+  }
+  ListNode* current = dummy.next;
+  while (!(list1 == nullptr && list2 == nullptr)) {
+    if (list1 == nullptr) {
+      current->next = list2;
+      current = current->next;
+      list2 = list2->next;
+    } else if (list2 == nullptr) {
+      current->next = list1;
+      current = current->next;
+      list1 = list1->next;
+    } else {
+      if (list1->val > list2->val) {
+        current->next = list2;
+        current = current->next;
+        list2 = list2->next;
+      } else {
+        current->next = list1;
+        current = current->next;
+        list1 = list1->next;
+      }
+    }
+  }
+  return dummy.next;
+}