Problem 2: add two numbers

problems/2-add-two-numbers/Makefile

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+../template/Makefile

problems/2-add-two-numbers/lib.c

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+#include <stdlib.h>
+#include "lib.h"
+
+struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {
+    struct ListNode* current = NULL;
+    struct ListNode* first_node = NULL;
+
+    int reminder = 0;
+    while (l1 != NULL || l2 != NULL || reminder != 0) {
+        // Read digits
+        int a = 0;
+        int b = 0;
+        if (l1 != NULL)
+            a = l1->val;
+        if (l2 != NULL)
+            b = l2->val;
+
+        // Move pointers to the next node
+        if (l1 != NULL)
+            l1 = l1->next;
+        if (l2 != NULL)
+            l2 = l2->next;
+
+        // Calculate the result
+        int digit = (a + b + reminder) % 10;
+        reminder = (a + b + reminder) / 10;
+
+        // Create the first node
+        if (current == NULL) {
+            current = malloc(sizeof(struct ListNode));
+            first_node = current;
+        }
+        // Create regular node
+        else {
+            current->next = malloc(sizeof(struct ListNode));
+            current = current->next;
+        }
+
+        // Save the result
+        current->val = digit;
+        current->next = NULL;
+    }
+    
+    return first_node;
+}

problems/2-add-two-numbers/lib.h

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+#pragma once
+
+struct ListNode {
+    int val;
+    struct ListNode *next;
+};
+
+struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2);

problems/2-add-two-numbers/main.c

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+#include <stdlib.h>
+#include <stdio.h>
+#include "lib.h"
+
+// Creates single node
+struct ListNode* mkNode(int val) {
+    struct ListNode* node = malloc(sizeof(struct ListNode));
+    node->val = val;
+    node->next = NULL;
+    return node;
+}
+
+// Creates linked list
+struct ListNode* mkList(int* nums, int length) {
+    struct ListNode* first = NULL;
+    struct ListNode* current = NULL;
+    for (int i=0; i<length; i++) {
+        // First node
+        if (current == NULL) {
+            current = mkNode(nums[i]);
+            first = current;
+        }
+        // Regular node
+        else {
+            current->next = mkNode(nums[i]);
+            current = current->next;
+        }
+    }
+    return first;
+}
+
+// Returns 1 if linked lists are equal.
+// Returns 0 otherwise.
+int cmpLists(struct ListNode* a, struct ListNode* b) {
+    while (a != NULL || b != NULL) {
+        // Both nodes exist
+        if (a == NULL || b == NULL)
+            return 0;
+
+        // Check values
+        if (a->val != b->val)
+            return 0;
+        
+        // Go to next node
+        a = a->next;
+        b = b->next;
+    }
+
+    return 1;
+}
+
+// Free allocated memory
+void freeLinkedList(struct ListNode* list) {
+    struct ListNode* current;
+    while (list != NULL) {
+        current = list;
+        list = list->next;
+        free(current);
+    }
+}
+
+int main() {
+    // First list
+    int a[] = {2, 4, 3};
+    struct ListNode* list_a = mkList(a, 3);
+
+    // Second list
+    int b[] = {5, 6, 4};
+    struct ListNode* list_b = mkList(b, 3);
+
+    // Expected sum
+    int c[] = {7, 0, 8};
+    struct ListNode* list_c = mkList(c, 3);
+
+    // Computed result
+    struct ListNode* res = addTwoNumbers(list_a, list_b);
+
+    printf("Test 1: %d\n", cmpLists(res, list_c));
+
+    freeLinkedList(list_a);
+    freeLinkedList(list_b);
+    freeLinkedList(list_c);
+    freeLinkedList(res);
+
+    return 0;
+}

problems/2-add-two-numbers/readme.md

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+# 2. Add two numbers
+
+* [Leetcode](https://leetcode.com/problems/add-two-numbers/)
+
+You are given two non-empty linked lists representing two non-negative integers.
+The digits are stored in reverse order, and each of their nodes contains a single digit.
+Add the two numbers and return the sum as a linked list.
+
+You may assume the two numbers do not contain any leading zero, except the number 0 itself.