Add Two Numbers

add-two-numbers.c

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {
    if (!l1) return l2;
    if (!l2) return l1;
    int carry = 0;
    struct ListNode* root = malloc(sizeof(struct ListNode*));
    struct ListNode* node = root;
    while (l1 || l2) {
        if (!l1 && carry == 0) {
            node->next = l2;
            break;
        }
        if (!l2 && carry == 0) {
            node->next = l1;
            break;
        }
        int sum = carry;
        if (l1) {
            sum += l1->val;
            l1 = l1->next;
        }
        if (l2) {
            sum += l2->val;
            l2 = l2->next;
        }
        carry = sum / 10;
        struct ListNode* newNode = malloc(sizeof(struct ListNode*));
        newNode->val = sum % 10;
        node->next = newNode;
        node = newNode;
    }
    if (carry > 0) {
        struct ListNode* newNode = malloc(sizeof(struct ListNode*));
        newNode->val = carry;
        node->next = newNode;
    }
    return root->next;
}