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

/**
 * 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 head = l1;
        int add = 0;
        ListNode p1 = null;
        ListNode p2 = null;
        boolean flag = false;
        while(l1 != null || l2 != null) {
            int temp = 0;
            if(l1 != null) {
                temp += l1.val;
                p1 = l1;
                l1 = l1.next;
            } else  if(!flag){
                flag = true;
                p1.next = l2;
            }
            if(l2 != null) {
                temp += l2.val;
                p2 = l2;
                l2 = l2.next;
            }
            temp += add;
            if(!flag) {
                p1.val = temp % 10;
            } else {
                p2.val = temp % 10;
            }
            add = temp / 10;
        }
        if(add == 1) {
            ListNode end = new ListNode(1);
            end.next = null;
            if(flag) {
                p2.next = end;
            } else {
                p1.next = end;
            }
        }
        return head;
    }
}

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;
            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;
    }
}