【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

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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;
    }
}