leetcode AddTwoNumbers 题解

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

题意分析：上面例子中实际指的是   342 + 465 = 807  只是把每个数的每一位给拆开了，用逆向的链表表示出来了。

/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }//next未声明，默认是null，如果val不声明，默认是0.
* }
*/
public class Solution {
/**
* 注意有进位的情况,这里的flag就是保存进位的情况
* 这里需要注意的是
* ListNode pre = new ListNode(0);
* ListNode head = pre;
* 这里的pre、head都是引用，指向的都是new ListNode(0)这个实例对象
*/

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
ListNode pre = new ListNode(0);
ListNode head = pre;
int flag = 0;
while(l1!=null||l2!=null||flag!=0){
if(l1!=null){
flag+=l1.val;
l1=l1.next;
}
if(l2!=null){
flag+=l2.val;
l2=l2.next;
}
ListNode cur = new ListNode(flag%10);
head.next = cur;
flag=flag/10;
head = head.next;
}
return pre.next;
}
}