🟢 Port problem 21 (Merge Two Sorted Lists) from TypeScript to C++23

solutions/0001-0100/0021-merge-two-sorted-lists/README.md

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+# 21. Merge Two Sorted Lists
+
+[LeetCode problem 21](https://leetcode.com/problems/merge-two-sorted-lists/)
+
+- **Difficulty**: Easy
+- **Tags**: linked-list, recursion
+
+## Description
+
+Given the heads of two sorted linked lists, merge them into one sorted list
+by splicing together the nodes of the two lists, and return the head of the
+merged list.

solutions/0001-0100/0021-merge-two-sorted-lists/solution.cpp

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+#include "solution.hpp"
+
+namespace leetcode::p0021 {
+
+ListNode* solve(ListNode* list1, ListNode* list2) {
+    ListNode dummy;
+    ListNode* current = &dummy;
+
+    while (list1 != nullptr && list2 != nullptr) {
+        if (list1->val <= list2->val) {
+            current->next = list1;
+            list1 = list1->next;
+        } else {
+            current->next = list2;
+            list2 = list2->next;
+        }
+        current = current->next;
+    }
+
+    current->next = (list1 != nullptr) ? list1 : list2;
+
+    return dummy.next;
+}
+
+} // namespace leetcode::p0021

solutions/0001-0100/0021-merge-two-sorted-lists/solution.hpp

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+#pragma once
+
+namespace leetcode::p0021 {
+
+struct ListNode {
+    int val;
+    ListNode* next;
+
+    explicit ListNode(int value = 0, ListNode* nextNode = nullptr) : val(value), next(nextNode) {}
+};
+
+// optimized: O(n + m) time, O(1) extra space — splice nodes onto a dummy
+// head, iteratively.
+ListNode* solve(ListNode* list1, ListNode* list2);
+
+} // namespace leetcode::p0021

solutions/0001-0100/0021-merge-two-sorted-lists/test.cpp

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+#include <vector>
+
+#include <catch2/catch_test_macros.hpp>
+
+#include "solution.hpp"
+
+namespace {
+
+leetcode::p0021::ListNode* fromVector(const std::vector<int>& values) {
+    leetcode::p0021::ListNode dummy;
+    leetcode::p0021::ListNode* current = &dummy;
+    for (int value : values) {
+        current->next = new leetcode::p0021::ListNode(value);
+        current = current->next;
+    }
+    return dummy.next;
+}
+
+std::vector<int> toVector(leetcode::p0021::ListNode* head) {
+    std::vector<int> result;
+    for (auto* node = head; node != nullptr; node = node->next) {
+        result.push_back(node->val);
+    }
+    return result;
+}
+
+} // namespace
+
+TEST_CASE("Problem 21", "[p0021]") {
+    CHECK(toVector(leetcode::p0021::solve(fromVector({1, 2, 4}), fromVector({1, 3, 4}))) ==
+          std::vector<int>{1, 1, 2, 3, 4, 4});
+    CHECK(toVector(leetcode::p0021::solve(fromVector({}), fromVector({}))).empty());
+    CHECK(toVector(leetcode::p0021::solve(fromVector({}), fromVector({0}))) == std::vector<int>{0});
+}