【LeetCode 2. Add Two Numbers】

Description

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.

翻译：

给你2个非空链表，代表2个非负整数。链表中每一位数字是反序存储的并且数组的每个节点包含一个单独的数字。把这两个非负整数相加并且用链表返回。

您可以假设除了数字 0 之外，这两个数都不会以 0 开头。

Kotlin版代码：

class ListNode constructor(var initVal : Int) {
    var next: ListNode? = null
}

fun addTwoNumbers(l1: ListNode?, l2: ListNode?): ListNode? {
    val head = ListNode(0)
    var p = l1
    var q = l2
    var currentNode: ListNode? = head
    var carry = 0
    while (p != null || q != null) {
        val x = p?.initVal ?: 0
        val y = q?.initVal ?: 0
        val sum = carry + x + y
        carry = sum / 10
        currentNode?.next = ListNode(sum % 10)
        currentNode = currentNode?.next
        if (p != null) p = p.next
        if (q != null) q = q.next
    }
    if (carry > 0) {
        currentNode?.next = ListNode(carry)
    }
    return head.next
}

参考：
https://leetcode.com/problems/add-two-numbers