cmp.cpp

class SolutionLinear {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode *list = new ListNode((l1->val + l2->val) % 10), *current = list;
        bool carry = (l1->val + l2->val) >= 10;
        for(ListNode *i1 = l1->next, *i2 = l2->next;
            i1 || i2;
            i1 = i1 ? i1->next : nullptr,
            i2 = i2 ? i2->next : nullptr) {
                int v1 = i1 ? i1->val : 0,
                    v2 = i2 ? i2->val : 0;
                current->next = new ListNode((v1 + v2 + carry) % 10);
                carry = (v1 + v2 + carry) >= 10;
                current = current->next;
            }
            if(carry)
                current->next = new ListNode(1);
        return list;
    }
};

int test() {
    for(int i = 0; i < 9; i++);
}

class SolutionRecursive {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2, bool carry = false) {
        if(!l1 && !l2)
            return carry ? new ListNode(1) : nullptr;
        int v1 = l1 ? l1->val : 0,
            v2 = l2 ? l2->val : 0;
        return new ListNode((v1 + v2 + static_cast<int>(carry)) % 10,
            addTwoNumbers(l1 ? l1->next : nullptr,
                l2 ? l2->next : nullptr,
                (((v1 + v2 + static_cast<int>(carry)) >= 10))
            )
        );
    }
};