Design a simplified version of Twitter where users can post tweets, follow/unfollow another user and is able to see the 10 most recent tweets in the user’s news feed. Your design should support the following methods:

1. postTweet(userId, tweetId): Compose a new tweet.
2. getNewsFeed(userId): Retrieve the 10 most recent tweet ids in the user’s news feed. Each item in the news feed must be posted by users who the user followed or by the user herself. Tweets must be ordered from most recent to least recent.
3. follow(followerId, followeeId): Follower follows a followee.
4. unfollow(followerId, followeeId): Follower unfollows a followee.

Example:

## leetcode Count of Range Sum

### leetcode Count of Range Sum

Given an integer array `nums`, return the number of range sums that lie in `[lower, upper]` inclusive.
Range sum `S(i, j)` is defined as the sum of the elements in `nums` between indices `i` and `j` (`i``j`), inclusive.

Note:
A naive algorithm of O(n2) is trivial. You MUST do better than that.

Example:
Given nums = `[-2, 5, -1]`, lower = `-2`, upper = `2`,
Return `3`.
The three ranges are : `[0, 0]`, `[2, 2]`, `[0, 2]` and their respective sums are: `-2, -1, 2`.

## leetcode Peeking Iterator

Given an Iterator class interface with methods: `next()` and `hasNext()`, design and implement a PeekingIterator that support the `peek()` operation — it essentially peek() at the element that will be returned by the next call to next()

Here is an example. Assume that the iterator is initialized to the beginning of the list: `[1, 2, 3]`.

Call `next()` gets you 1, the first element in the list.

Now you call `peek()` and it returns 2, the next element. Calling `next()` after that still return 2.

You call `next()` the final time and it returns 3, the last element. Calling `hasNext()` after that should return false.

Hint:

1. Think of “looking ahead”. You want to cache the next element.
2. Is one variable sufficient? Why or why not?
3. Test your design with call order of `peek()` before `next()` vs `next()` before `peek()`.
4. For a clean implementation, check out Google’s guava library source code.

Follow up: How would you extend your design to be generic and work with all types, not just integer?

## leetcode Minimum Height Trees

### leetcode Minimum Height Trees

For a undirected graph with tree characteristics, we can choose any node as the root. The result graph is then a rooted tree. Among all possible rooted trees, those with minimum height are called minimum height trees (MHTs). Given such a graph, write a function to find all the MHTs and return a list of their root labels.

Format
The graph contains `n` nodes which are labeled from `0` to `n - 1`. You will be given the number `n` and a list of undirected `edges` (each edge is a pair of labels).

You can assume that no duplicate edges will appear in `edges`. Since all edges are undirected, `[0, 1]` is the same as `[1, 0]` and thus will not appear together in `edges`.

Example 1:

Given `n = 4`, `edges = [[1, 0], [1, 2], [1, 3]]`

return ``

## leetcode Range Sum Query – Mutable

### leetcode Range Sum Query – Mutable

Given an integer array nums, find the sum of the elements between indices i and j (ij), inclusive.

The update(i, val) function modifies nums by updating the element at index i to val.

Example:

Given nums = [1, 3, 5]

sumRange(0, 2) -> 9

update(1, 2)

sumRange(0, 2) -> 8

Note:

1. The array is only modifiable by the update function.
2. You may assume the number of calls to update and sumRange function is distributed evenly.

• push
• pop
• 堆的创建及其他应用

## 算法复习 – 图论 最短路和最小生成树

• 最短路径
• dijkstra
• SPFA
• floyd
• 用SPFA解决差分约束
• 最小生成树MST
• prim
• kruskal
• 次小生成树
• 二分图
• 最大匹配和最小覆盖

## leetcode 数据结构

• 20. Valid Parentheses
• 146 LRU Cache
• 150 Evaluate Reverse Polish Notation
• 155 Min Stack
• 187 Repeated DNA Sequences
• 208 Implement Trie (Prefix Tree)
• 211 Add and Search Word – Data structure design
• 218 The Skyline Problem
• 232 Implement Queue using Stacks
• 239 Sliding Window Maximum
• 341 Flatten Nested List Iterator
• 352 Data Stream as Disjoint Intervals
• 432 All O’one Data Structure