# 445.Add-Two-Numbers-II

## 445. Add Two Numbers II

## 题目地址

<https://leetcode.com/problems/add-two-numbers-ii/>

## 题目描述

```
You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first 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.

Follow up:
What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example:

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7
```

## 代码

### Approach #1 Two Stack

```java
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
      Stack<Integer> s1 = new Stack<Integer>();
      Stack<Integer> s2 = new Stack<Integer>();

      while (l1 != null) {
        s1.push(l1.val);
        l1 = l1.next;
      }

      while (l2 != null) {
        s2.push(l2.val);
        l2 = l2.next;
      }

      int sum = 0;
      ListNode head = new ListNode(0);
      while (!s1.empty() || !s2.empty()) {
        if (!s1.empty())    sum += s1.pop();
        if (!s2.empty())    sum += s2.pop();
        head.val = sum % 10;
        ListNode node = new ListNode(sum / 10);
        node.next = head;
        head = node;
        sum /= 10;
      }

      return head.val == 0 ? head.next : head;
    }
}
```


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