445. 两数相加 II

题目链接：https://leetcode-cn.com/problems/add-two-numbers-ii/submissions/
解题思路：
 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) { val = x; }
 7  * }
 8  */
 9 class Solution {
10     public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
11     Stack<Integer> l1Stack = buildStack(l1);
12     Stack<Integer> l2Stack = buildStack(l2);
13     ListNode head = new ListNode(-1);
14     ListNode p =head;
15     int carry = 0;
16     int x;
17     int y;
18     while (!l1Stack.isEmpty() || !l2Stack.isEmpty() || carry != 0) {
19         if(!l1Stack.isEmpty())
20             x = l1Stack.pop();
21         else
22             x=0;
23         if(!l2Stack.isEmpty())
24             y = l2Stack.pop();
25         else
26             y=0;
27         int sum = x + y + carry;
28         ListNode node = new ListNode(sum % 10);
29         node.next = head.next;
30         head.next=node;
31         carry = sum / 10;
32     }
33     return head.next;
34 }
35 
36 private Stack<Integer> buildStack(ListNode l) {
37     Stack<Integer> stack = new Stack<>();
38     while (l != null) {
39         stack.push(l.val);
40         l = l.next;
41     }
42     return stack;
43 }
44 }