Leetcode 2 两数相加

题目地址：https://leetcode-cn.com/problems/add-two-numbers/

难度：简单

基本思路：

• 大于10要考虑进位，具体进几位用整除，当前结点大小用模除
• 要考虑两条链表不一样长度的情况，所以遍历的判断条件是有一个不为空则继续，二者皆空则停止

class Solution {
    // 与官方题解思路一致, 效率低是因为多创建了结点
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode head = null, res = null;
        int carry = 0, val, l1Val, l2Val, sum;
        while(l1 != null || l2 != null){
            l1Val = l1 == null ? 0 : l1.val;
            l2Val = l2 == null ? 0 : l2.val;
            sum = l1Val + l2Val + carry;
            val = sum % 10;
            carry = sum / 10;
            if(head == null){
                res = head = new ListNode(val);
            }else{
                res.next = new ListNode(val);
                res = res.next;
            }
            if(l1 != null){
                l1 = l1.next;
            }
            if(l2 != null){
                l2 = l2.next;
            }
            if(carry > 0){
                res.next = new ListNode(carry);
            }
        }
        return head;
    }
}