You are given two linked lists representing two non-negative numbers. 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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

直接按位相加即可，最后拼接剩余的数字。

public class Solution {
  public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    ListNode head = new ListNode(0);
    ListNode l = head;
    
    int carry = 0;
    
    while (l1 != null || l2 != null) {
        int value = 0;
        if (l1 != null) {
            value += l1.val;
            l1 = l1.next;
        }
        if (l2 != null) {
            value += l2.val;
            l2 = l2.next;
        }
        l.next = new ListNode((value+carry)%10);
        l = l.next;
        carry = (value+carry)/10;
    }
    if (carry != 0) {
        l.next = new ListNode(carry);
    }
    return head.next;
  }
}