New test infrastructure for Linked List/ Graph problems in Leetcode

[xxks-kkk]

Leetcode now adopts a new way to represent linked list and graph input. For example, in LC138, the input looks like:

Input:
{"$id":"1","next":{"$id":"2","next":null,"random":{"$ref":"2"},"val":2},"random":{"$ref":"2"},"val":1}

Now, we need to have infrastructure support for creating linked list and graph based on the new input format.

This issue is critical as now linked list and graph problem cannot be properly tested locally.