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

https://oj.leetcode.com/problems/add-two-numbers/

思路：模拟题，从头向后依次遍历两个链表相加。

注意：1. 用dummy head简便处理。2. 注意carry的处理，尤其最高位进位的情况。3. 注意引用null的判断。

public class Solution {    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {        // printList(l1);        // printList(l2);        ListNode p = new ListNode(-1);        ListNode head = p;        int c = 0;        while (l1 != null || l2 != null || c == 1) {            int curInt = 0;            if (l1 != null)                curInt += l1.val;            if (l2 != null)                curInt += l2.val;            curInt += c;            if (curInt > 9) {                c = 1;                curInt -= 10;            } else                c = 0;            p.next = new ListNode(curInt);            p = p.next;            if (l1 != null)                l1 = l1.next;            if (l2 != null)                l2 = l2.next;        }        // printList(head.next);        return head.next;    }    void printList(ListNode node) {        while (node != null) {            System.out.print(node.val);            if (node.next != null)                System.out.print("->");            node = node.next;        }        System.out.println();    }    public static void main(String[] args) {        ListNode l1 = new ListNode(2);        l1.next = new ListNode(4);        l1.next.next = new ListNode(5);        ListNode l2 = new ListNode(5);        l2.next = new ListNode(6);        l2.next.next = new ListNode(4);        new Solution().addTwoNumbers(l1, l2);    }}class ListNode {    int val;    ListNode next;    ListNode(int x) {        val = x;        next = null;    }}