# 2     Add Two Numbers

## 2. [Add Two Numbers](https://leetcode.com/problems/add-two-numbers/description/)

## 1. Question

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.

**Example**

```
Input:(2 -> 4 -> 3) + (5 -> 6 -> 4)

Output:7 -> 0 -> 8

Explanation: 342 + 465 = 807.
```

## 2. Implementation

```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) {
        ListNode p1 = l1, p2 = l2;
        ListNode dummy = new ListNode(0);
        ListNode curNode = dummy;
        int sum = 0;

        while (p1 != null || p2 != null) {
            if (p1 != null) {
                sum += p1.val;
                p1 = p1.next;
            }

            if (p2 != null) {
                sum += p2.val;
                p2 = p2.next;
            }

            curNode.next = new ListNode(sum % 10);
            curNode = curNode.next;
            sum /= 10;
        }

        if (sum != 0) {
            curNode.next = new ListNode(sum);
        }
        return dummy.next;
    }
}
```

## 3. Time & Space Complexity

时间复杂度O(m + n), m为第一个list的长度，n为第二个list的长度， 空间复杂度O(1)
