Add Two Numbers

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

只要最后检查下进位是不是1。

public class Solution {
    //Time: O(n)
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }
        
        int carry = 0;
        ListNode dummy = new ListNode(0);
        ListNode node = dummy;
        while (l1 != null || l2 != null) {
            if (l1 != null) {
                carry += l1.val;
                l1 = l1.next;
            }
            if (l2 != null) {
                carry += l2.val;
                l2 = l2.next;
            }
            node.next = new ListNode(carry % 10);
            carry /= 10;
            node = node.next;
        }
        if (carry == 1) {
            node.next = new ListNode(1);
        }
        return dummy.next;
    }
}