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;
}
}