/**
 * 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) {
        int carry = 0;
        ListNode last = null;
        ListNode current = null;
        ListNode result = null;

        while(l1!=null && l2!=null){
            if(l1.val+l2.val + carry >= 10){
                current = new ListNode(l1.val+l2.val+carry - 10);
                carry = 1;
                
            }else{
                current = new ListNode(l1.val+l2.val+carry);
                carry = 0;
                
            }
            
            if(last == null){
                result = current;
            }else{
                last.next = current;
            }
            
            
            l1 = l1.next;
            l2 = l2.next;
            if(l1 == null&&l2==null &&carry == 1){
                current.next = new ListNode(1);
                break;
            }
            last = current;
        }
        
        while(l1!=null){
            if(l1.val + carry >= 10){
                current = new ListNode(l1.val +carry - 10);
                carry = 1;
            }else{
                current = new ListNode(l1.val + carry);
                carry = 0;
            }
            
                
            last.next = current;
            last = current;
            l1 = l1.next;
            if(l1 == null && carry == 1){
                current.next = new ListNode(1);
                break;
            }
        }
        while(l2!=null){
            if(l2.val + carry >= 10){
                current = new ListNode(l2.val +carry - 10);
                carry = 1;
            }else{
                current = new ListNode(l2.val + carry);
                carry = 0;
            }
            last.next = current;
            last = current;
            l2 = l2.next;
            if(l2 == null && carry == 1){
                current.next = new ListNode(1);
                break;
            }
        }
        
        return result;
    }
}

Optimization~~~

public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode c1 = l1;
        ListNode c2 = l2;
        ListNode sentinel = new ListNode(0);
        ListNode d = sentinel;
        int sum = 0;
        while (c1 != null || c2 != null) {
            sum /= 10;  //goode coding  to present the addtiong digit
            if (c1 != null) {
                sum += c1.val;
                c1 = c1.next;
            }
            if (c2 != null) {
                sum += c2.val;
                c2 = c2.next;
            }
            d.next = new ListNode(sum % 10);
            d = d.next;
        }
        if (sum / 10 == 1)
            d.next = new ListNode(1);
        return sentinel.next;
    }
}