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# \[LeetCode] 2. Add Two Numbers \[Medium]

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.
```

**Analyse:**

兩個List的Node依序相加, 若結果大於10則記錄需進位.

**Solution:** ↩

```swift
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.next = nil
 *     }
 * }
 */
class Solution {

    func addTwoNumbers(_ l1: ListNode?, _ l2: ListNode?) -> ListNode? {   
        var node1 = l1;
        var node2 = l2;
        var output = ListNode(0);
        var head = output;
        
        var carry = 0;
        
        while(node1 != nil || node2 != nil) {
            var sum = carry;
            
            if(node1 != nil) {
                sum = sum + node1!.val;
                node1 = node1!.next
            }
            
            if(node2 != nil) {
                sum = sum + node2!.val;
                node2 = node2!.next
            }
            
            let value = sum % 10;
            carry = sum / 10;
            
            var newNode = ListNode(value);
            output.next = newNode;
            output = output.next!;
        }
        
        if(carry != 0) {
            var newNode = ListNode(carry);
            output.next = newNode;
            output = output.next!;
        }
        
        return head.next;
    }
}
```