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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

Solution:

In each round, we do:
1. Create a node with the value of the sum of heads of l1 and l2.
2. Move the heads of l1 and l2 to their next.
3. Update carry.
4. Connect the node to the newly created linked list. 

Code:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode curr = dummy;
        int carry = 0;
        while (l1 != null || l2 != null || carry == 1) {
            int v1 = 0;
            int v2 = 0;
            if (l1 != null) {
                v1 = l1.val;
                l1 = l1.next;
            }
            if (l2 != null) {
                v2 = l2.val;
                l2 = l2.next;
            }
            int sum = v1 + v2 + carry;
            carry = sum >= 10 ? 1 : 0;
            curr.next = new ListNode(sum % 10);
            curr = curr.next;
        }
        return dummy.next;
    }
}