feat(src): add 21-mergeTwoLists

Author: kwu130

src/21-mergeTwoLists.cpp

@@ -0,0 +1,61 @@
+//
+// Created by 1412 on 2025/10/16.
+//
+
+#include <bits/stdc++.h>
+#include "../include/common.h"
+
+using namespace std;
+using listHelper::ListNode;
+
+class Solution {
+public:
+    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
+        ListNode* dummy = new ListNode(-1);
+        ListNode* cur = dummy;
+        while (list1 && list2) {
+            if (list1->val < list2->val) {
+                cur->next = list1;
+                list1 = list1->next;
+            } else {
+                cur->next = list2;
+                list2 = list2->next;
+            }
+            cur = cur->next;
+        }
+        cur->next = list1 ? list1 : list2;
+        return dummy->next;
+    }
+};
+
+void test() {
+    Solution s;
+    ListNode* l1 = listHelper::build({1, 2, 4});
+    ListNode* l2 = listHelper::build({1, 3, 4});
+    ListNode* expected = listHelper::build({1, 1, 2, 3, 4, 4});
+    ListNode* res = s.mergeTwoLists(l1, l2);
+    assert(listHelper::equal(res, expected));
+    listHelper::free(res);
+    listHelper::free(expected);
+
+    l1 = listHelper::build({});
+    l2 = listHelper::build({});
+    expected = listHelper::build({});
+    res = s.mergeTwoLists(l1, l2);
+    assert(listHelper::equal(res, expected));
+    listHelper::free(res);
+    listHelper::free(expected);
+
+    l1 = listHelper::build({});
+    l2 = listHelper::build({0});
+    expected = listHelper::build({0});
+    res = s.mergeTwoLists(l1, l2);
+    assert(listHelper::equal(res, expected));
+    listHelper::free(res);
+    listHelper::free(expected);
+}
+
+int main() {
+    test();
+    return 0;
+}