2. 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 contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) { // time: O(n1 + n2)
    ListNode* dummy = new ListNode(-1), *cur = dummy;
    int carry = 0;
    while (l1 || l2) {
        int num = 0;
        if (l1) {
            num += l1->val;
            l1 = l1->next;
        }
        if (l2) {
            num += l2->val;
            l2 = l2->next;
        }
        num += carry;
        cur->next = new ListNode(num % 10);
        cur = cur->next;
        carry = num / 10;
    }
    if (carry) cur->next = new ListNode(carry);
    ListNode* res = dummy->next;
    delete dummy;
    return res;
}