2. Add Two Numbers

题目：

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 contain a single digit. Add the two numbers and return it as a linked list.

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

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

链接：https://leetcode.com/problems/add-two-numbers/#/description

4/3/2017

51ms, 88%

public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int carry = 0;
        ListNode ret = new ListNode(-1);
        ListNode dummy = ret;
        int sum = 0;

        while(l1 != null || l2 != null) {
            if (l1 != null && l2 != null) {
                sum = (l1.val + l2.val + carry) % 10;
                carry = (l1.val + l2.val + carry) / 10;
                l1 = l1.next;
                l2 = l2.next;
            } else if (l1 != null) {
                sum = (l1.val + carry) % 10;
                carry = (l1.val + carry) / 10;     
                l1 = l1.next;
            } else {
                sum = (l2.val + carry) % 10;
                carry = (l2.val + carry) / 10;            
                l2 = l2.next;
            }
            ret.next = new ListNode(sum);
            ret = ret.next;
        }
        if (carry == 1) {
            ret.next = new ListNode(carry);
        }
        return dummy.next;
    }
}

超时，难道就因为test case里有长度相差很大的2个list吗？但是看起来跟答案一样啊

public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int carry = 0;
        ListNode ret = new ListNode(-1);
        ListNode dummy = ret;
        int value;

        while(l1 != null || l2 != null) {
            value = carry;
            if (l1 != null) {
                value += l1.val;
                l1 = l1.next;
            }
            if (l1 != null) {
                value += l2.val;
                l2 = l2.next;
            }
            carry = value / 10;
            ret.next = new ListNode(value % 10);
            ret = ret.next;
        }
        if (carry == 1) {
            ret.next = new ListNode(carry);
        }
        return dummy.next;
    }
}

解答：https://leetcode.com/articles/add-two-numbers/