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

 

复杂度：

时间：O(n+m)、空间：O(1)

 

实现：

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */public class Solution {    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {        ListNode headNode = null, preNode = null;        int carry = 0;        while(l1 != null || l2 != null || carry > 0) {            int sum = (l1 != null ? l1.val : 0) + (l2 != null ? l2.val : 0) + carry;            carry = sum / 10;                        ListNode curNode = new ListNode(sum % 10);            if(preNode == null) {                headNode = curNode;            } else {                preNode.next = curNode;            }            preNode  = curNode;                        if(l1 != null && l1.next != null) {                l1 = l1.next;            } else {                l1 = null;            }                        if(l2 != null && l2.next != null) {                l2 = l2.next;            } else {                l2 = null;            }        }                return headNode;    }}