🟡 Port problem 2 (Add Two Numbers) from TypeScript to C++23

solutions/0001-0100/0002-add-two-numbers/README.md

@@ -0,0 +1,12 @@
+# 2. Add Two Numbers
+
+[LeetCode problem 2](https://leetcode.com/problems/add-two-numbers/)
+
+- **Difficulty**: Medium
+- **Tags**: linked-list, math, recursion
+
+## Description
+
+Given two non-empty linked lists representing two non-negative integers, the
+digits stored in reverse order, add the two numbers and return the sum as a
+linked list in the same reversed-digit format.

solutions/0001-0100/0002-add-two-numbers/solution.cpp

@@ -0,0 +1,29 @@
+#include "solution.hpp"
+
+namespace leetcode::p0002 {
+
+ListNode* solve(ListNode* l1, ListNode* l2) {
+    ListNode dummy;
+    ListNode* current = &dummy;
+    int carry = 0;
+
+    while (l1 != nullptr || l2 != nullptr || carry != 0) {
+        int sum = carry;
+        if (l1 != nullptr) {
+            sum += l1->val;
+            l1 = l1->next;
+        }
+        if (l2 != nullptr) {
+            sum += l2->val;
+            l2 = l2->next;
+        }
+
+        carry = sum / 10;
+        current->next = new ListNode(sum % 10);
+        current = current->next;
+    }
+
+    return dummy.next;
+}
+
+} // namespace leetcode::p0002

solutions/0001-0100/0002-add-two-numbers/solution.hpp

@@ -0,0 +1,16 @@
+#pragma once
+
+namespace leetcode::p0002 {
+
+struct ListNode {
+    int val;
+    ListNode* next;
+
+    explicit ListNode(int value = 0, ListNode* nextNode = nullptr) : val(value), next(nextNode) {}
+};
+
+// optimized: O(max(n, m)) time, O(max(n, m)) space — build the result list
+// directly while walking both lists and carrying, in one pass.
+ListNode* solve(ListNode* l1, ListNode* l2);
+
+} // namespace leetcode::p0002

solutions/0001-0100/0002-add-two-numbers/test.cpp

@@ -0,0 +1,37 @@
+#include <vector>
+
+#include <catch2/catch_test_macros.hpp>
+
+#include "solution.hpp"
+
+namespace {
+
+leetcode::p0002::ListNode* fromVector(const std::vector<int>& values) {
+    leetcode::p0002::ListNode dummy;
+    leetcode::p0002::ListNode* current = &dummy;
+    for (int value : values) {
+        current->next = new leetcode::p0002::ListNode(value);
+        current = current->next;
+    }
+    return dummy.next;
+}
+
+std::vector<int> toVector(leetcode::p0002::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 2", "[p0002]") {
+    CHECK(toVector(leetcode::p0002::solve(fromVector({2, 4, 3}), fromVector({5, 6, 4}))) ==
+          std::vector<int>{7, 0, 8});
+    CHECK(toVector(leetcode::p0002::solve(fromVector({0}), fromVector({0}))) ==
+          std::vector<int>{0});
+    CHECK(toVector(leetcode::p0002::solve(fromVector({9, 9, 9, 9, 9, 9, 9}),
+                                          fromVector({9, 9, 9, 9}))) ==
+          std::vector<int>{8, 9, 9, 9, 0, 0, 0, 1});
+}