Problem

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.

即合并两个已经排好序的链表。并且要求返回的新链表是由原两个链表元素组合而成的。链表的结构定义也已经固定给出：

public class ListNode {
        int val;
        ListNode next;

        ListNode(int x) {
            val = x;
        }
    }

Problem

Given a string containing just the characters ‘(‘, ‘)’, ‘{‘, ‘}’, ‘[’ and ‘]’, determine if the input string is valid.

The brackets must close in the correct order, “()” and “()[]{}” are all valid but “(]” and “([)]” are not.

即判断一个只包含括号的字符串，是不是合法的。

Problem

Write a function to find the longest common prefix string amongst an array of strings.

就是找出一个字符串数组中元素，最长的通用前缀。例如：{“ab”,”abc”,”abd”}，答案是 “ab”。

Problem

Given an integer, convert it to a roman numeral.

Input is guaranteed to be within the range from 1 to 3999.

跟Roman To Integer是对应问题。即将整数转换成对应的罗马数字。罗马数字规则见wiki：罗马数字规则

Problem

Given a roman numeral, convert it to an integer.

Input is guaranteed to be within the range from 1 to 3999.

即将罗马数字1-3999转换成对应的整数。罗马数字规则见wiki：罗马数字规则

Problem

Determine whether an integer is a palindrome. Do this without extra space.

即返回一个数是否是回数。例如：1，1221，12321是回数，负数不是回数。题目特别提醒，不要使用额外的空间，即不要考虑转换成String来处理。

Problem

Reverse digits of an integer.

Example1: x = 123, return 321 Example2: x = -123, return -321

Note: The input is assumed to be a 32-bit signed integer. Your function should return 0 when the reversed integer overflows.

即返回一个数的逆序数。

研究背景

上篇(Httpcomponents-core 从池中获取连接代码解析)研究中，我们知道，Httpcomponents-client底层维护了一个socket连接池，对于同一个地址，默认只可以建立2个连接。如果连接不及时释放，就会造成连接池中无可用的连接获取，从而导致请求等待（阻塞）。因此，我们需要关注连接池释放的逻辑。

最近使用Httpcomponents-client过程中遇到一个线程阻塞的问题。通过jstack dump线程发现，是block在AbstractConnPool的getPoolEntryBlocking中，于是决定研究一下Httpcomponents-client的源码。

刚发布的Java9中，有一个特性是新增了快速构造不可变集合的工厂函数。官方说明如下：

JEP 269: Convenience Factory Methods for Collections

Makes it easier to create instances of collections and maps with small numbers of elements. New static factory methods on the List, Set, and Map interfaces make it simpler to create immutable instances of those collections.

For example: Set alphabet = Set.of("a", "b", "c");

See Creating Immutable Lists, Sets, and Maps in Java Platform, Standard Edition Java Core Libraries Developer’s Guide. For API documentation, see Immutable Set Static Factory Methods, Immutable Map Static Factory Methods, and Immutable List Static Factory Methods.