[Leetcode] 2. Add Two Numbers 两数

Related Topics：[Linked List][Math]
Similar Questions
[Multiply Strings][Add Binary][Sum of Two Integers][Add Strings][Add Two Numbers II]

题目：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

思路
建立一个新链表，然后把输入的两个链表从头往后撸，每两个相加，添加一个新节点到新链表后面，就是要处理下进位问题。还有就是最高位的进位问题要最后特殊处理一下。

java解法：

/**
 * 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) {
        ListNode dummy=new ListNode(-1);
        ListNode cur=dummy;
        int carry=0;
        while((l1!=null)||(l2!=null)) {
            //要注意两个链表的长度不同的情况，当链表为null时，设置为零
            int d1=(l1==null? 0:l1.val);
            int d2=(l2==null? 0:l2.val);
            int sum=d1+d2+carry;
            //carry=sum/10;
            carry=sum>=10? 1:0;
            cur.next=new ListNode(sum%10);
            cur=cur.next;
            //null无法链接next
            if(l1!=null) l1=l1.next;
            if(l2!=null) l2=l2.next;
        }
        if(carry>0) cur.next=new ListNode(carry);
        return dummy.next;
    }
}