無聊刷LeetCode系列 2. Add Two Numbers

You are given two linked lists representing two non-negative numbers. 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.

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

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