【leetcode刷题笔记】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

题解：模拟即可。用carries保存进位，当l1和l2一方为空的时候，余下不为空的链表要单独处理。当l1和l2都为空的时候，如果carries不为空，那么要再单独申请一个链表存放carries，代码如下：

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) {
*         val = x;
*         next = null;
*     }
* }
*/
public class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
if(l1 == null && l2 == null)
return null;

ListNode kepeler = new ListNode(0);
ListNode head = kepeler;
int carries = 0;

while(l1 != null && l2 != null){
int sum = carries + l1.val + l2.val;
carries = sum/10;
kepeler.next = new ListNode(sum%10);
kepeler = kepeler.next;
l1 = l1.next;
l2 = l2.next;
}

while(l1 != null){
int sum = carries + l1.val;
carries = sum/10;
kepeler.next = new ListNode(sum%10);
l1 = l1.next;
kepeler = kepeler.next;
}

while(l2 != null){
int sum = carries + l2.val;
carries = sum/10;
kepeler.next = new ListNode(sum%10);
l2 = l2.next;
kepeler = kepeler.next;
}

if(carries != 0)
kepeler.next = new ListNode(carries);

return head.next;
}
}