Leetcode solution 1344. Angle Between Hands of a Clock

Problem Statement 

Given two numbers, hour and minutes. Return the smaller angle (in degrees) formed between the hour and the minute hand.

 

Example 1:

Input: hour = 12, minutes = 30
Output: 165

Example 2:

Input: hour = 3, minutes = 30
Output: 75

Example 3:

Input: hour = 3, minutes = 15
Output: 7.5

Example 4:

Input: hour = 4, minutes = 50
Output: 155

Example 5:

Input: hour = 12, minutes = 0
Output: 0

 

Constraints:

• 1 <= hour <= 12
• 0 <= minutes <= 59
• Answers within 10^-5 of the actual value will be accepted as correct.

 Problem link

Video Tutorial

You can find the detailed video tutorial here

• Youtube
  
• B站

Thought Process

Purely a math problem. Calculate the clock's hour hand and minute hand separately.

• There are 60 minutes in 360 angle, so each minute is 6 degree in angle.
• Angle should be the absolute value of (minute angel - hour angle)
• Final angle should be min(angle, 360 - angle)
  

Solutions

Time Complexity: O(1) since it's a math problem
Space Complexity: O(1) no extra space is used

References

• Leetcode official solution
  

Leetcode solution 1512. Number of Good Pairs

Problem Statement 

Given an array of integers nums.

A pair (i,j) is called good if nums[i] == nums[j] and i < j.

Return the number of good pairs.

 

Example 1:

Input: nums = [1,2,3,1,1,3]
Output: 4
Explanation: There are 4 good pairs (0,3), (0,4), (3,4), (2,5) 0-indexed.

Example 2:

Input: nums = [1,1,1,1]
Output: 6
Explanation: Each pair in the array are good.

Example 3:

Input: nums = [1,2,3]
Output: 0

 

Constraints:

• 1 <= nums.length <= 100
• 1 <= nums[i] <= 100

 Problem link

Video Tutorial

You can find the detailed video tutorial here

• Youtube
  
• B站

Thought Process

Array is the most basic data structures and common solutions inclue

• Nested for loops O(N^2)
• Sort O(NlgN)
• Use extra space O(N)

This applies to this problem perfectly. The O(N^2) brute force solution is naive. We can also use a Map data structure (key is the number, value is the occurrence count) thus O(N). We can also sort the array and use this simple formula (also leetcode's hint) to calculate the good number pair.

Good pairs = N * (N-1) / 2 where N is how many duplicate numbers, this is from combination C(n^2), from n elements pick two

Solutions

Use Map

Time Complexity: O(N), N is the array size
Space Complexity: O(N) since we use extra Map

Sort

Time Complexity: O(NlgN), N is the array size since we sort
Space Complexity: O(1) no extra space is used

References

• None
  

System design interview: how to design a simple twitter search system

Methodology: READ MF!

[Originally from the Post: System design interview: how to design a chat system (e.g., Facebook Messenger, WeChat or WhatsApp)]

Remind ourselves with the "READ MF!" methodology. Update: 08/2025 "BREAD MF!" 

Requirements

Simple text based search with "AND" or "OR" support. E.g., give me all the tweets that have words "Black" AND "Life" in it sorted by published time in descending order.

search(api_dev_key, search_terms, maximum_results_to_return, sort, page_token)

A follow up question would be sort those results in other orders, E.g., give me all the tweets that have words "Black" AND "Life" in it sorted by the most commented tweets in descending order.

Estimation

• Storage: assuming twitter has 1B users, 50% of them post a tweet per day, each tweet is 140 char = 140 Byte. Each day, 1B * 0.5 * 280(roughly 300)Byte = 150GB data
• Write QPS = 1B user * 0.5 post a tweet per day / 86400 = 5787 QPS = 6000 QPS
  
• Query QPS = assuming 3x than write QPS = 18000 QPS
  

Write QPS in general we don't worry too much since it could be processed asynchronously. 18000 QPS for read is easy to handle as long as we have enough number of boxes and most of query would be returned via memory (not hitting disk). 

Key designs and terms

You can also divided the system as usual into three major layers, presentation, service and data layer.

A few key terminologies.

• Lucene
  
• Inverted Index
• Elastic Search Under the Hood,  Elastic Search 101
  
• Elastic Search API
• ELK
  

If the follow up question, we just need to build another index given the sort key or we could fetch the results on flight and sort (local optimal). Follow "Index -> Search -> Rank" steps.

  Baozi Youtube Video

• Youtube Video Link
• B站

References (Credits to original authors)

• Grokking the system design interviews
• Elastic Search Under the Hood GDrive
• Elastic Search Under the Hood source