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.

Idea – 1

We add the lists digit by digit in time.

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

Runtime: 2 ms, faster than 97.82% of Java online submissions for Add Two Numbers. Memory Usage: 40.7 MB, less than 73.62% of Java online submissions for Add Two Numbers.