*LeetCode--Add Two Numbers

Add Two Numbers   

两数之和，本位和进位的求和

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.

/**
 * 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) {
        if(l1 == null || l2 == null) return null;
        ListNode sum = new ListNode(0);
        ListNode node = sum;
        int num = 0;
        int carry = 0;
        while(l1 != null || l2 != null){    
            if(l1 != null) {
                num += l1.val;
                l1 = l1.next;
            }
            if(l2 != null) {
                num += l2.val;
                l2 = l2.next;
            }
            carry = num / 10;
            node.next = new ListNode(num % 10);
            node = node.next;
            num = carry;
        }
        if(num != 0) node.next = new ListNode(num);
        return sum.next;
    }
}