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 contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example 1:

Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(-1);
        ListNode cur = dummy;

        int carry = 0;
        while(l1 != null || l2 != null){ //这里是或的关系
            int x = l1 == null ? 0 : l1.val; // 要特别注意处理空的情况
            int y = l2 == null ? 0 : l2.val;

            int sum = x + y + carry;

            carry = sum/10;
            int cur_next_value = sum%10;
            
            if(l1 != null){
                l1 = l1.next;
            }

            if(l2 != null){
                l2 = l2.next;
            }

            ListNode cur_next = new ListNode(cur_next_value);
            cur.next = cur_next;
            cur = cur.next;
        }

        if(carry != 0){
            cur.next = new ListNode(carry); //这里要特别注意别忘了加上carry
        }

        return dummy.next;
    }
}