138. Copy List with Random Pointer
A linked list is given such that each node contains an additional random pointer which could point to any node in the list or null.
Return a deep copy of the list.
Java中的深拷贝：对象中的对象需要重写clone方法，将所有基本类型重新clone
109. Convert Sorted List to Binary Search Tree
Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.
Example:
Given the sorted linked list: [-10,-3,0,5,9], One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:
21. Merge Two Sorted Lists Easy
Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.
Example: Input: 1->2->4, 1->3->4 Output: 1->1->2->3->4->4
这是个Easy级别的，有前两次的经验，一次做对，但执行效率不高
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1021. Remove Outermost Parentheses
A valid parentheses string is either empty (""), “(” + A + “)”, or A + B, where A and B are valid parentheses strings, and + represents string concatenation. For example, “”, “()”, “(())()”, and “(()(()))” are all valid parentheses strings.
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1047. Remove All Adjacent Duplicates In String
Given a string S of lowercase letters, a duplicate removal consists of choosing two adjacent and equal letters, and removing them.
