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Problem Solving

Coding Interview Preparation

1. Asymptotic Notations

Big O Complexity Chart

time complexity graph

1 < log(n) < n < n*log(n) < n^2 < n^3 < ... < 2^n < 3^n < ... < n^n

<Polynomial>
    1 < log(n) < n < n*log(n) < n^2 < n^3 ...
</Polynomial>  

<Exponential>
    2^n < 3^n < ... < n^n
</Exponential>
  1. Lower Bound - Ω Omega
  2. Upper Bound - O Big-O
  3. Tight Bound - Θ Theta

Big-O Cheat Sheet

2. Heap - Heap Sort - Heapify - Priority Queues

3. Recursion

4. Dynamic Programming

5. Array and Strings

6. Trees and Graphs

7. 0/1 Knapsack

Unbounded knapsack 
Fractional Greedy
Memoization
Top Down

8. Knapsack variation Problem

8.1. Subset Sum Problem ~> [Link](https://www.geeksforgeeks.org/subset-sum-problem-dp-25/)
8.2. Equal Subset Sum Partition ~> [Link](https://www.youtube.com/watch?v=UmMh7xp07kY)
8.3. Count of Subset Sum
8.4. Minimum Subset Sum Difference
8.5. Number of Subset by a given Difference
8.6. Target Sum DP

9. Backtracking Problems

10. Searching & Sorting

10.1. How do I identify cyclic sort pattern?
      - They will be problems involving a sorted array with numbers in a given range
      - If the problem asks you to find the missing/duplicate/smallest number in an sorted/rotated array

11. System Design

11.1. [System Design Primer](https://github.com/tahmid-tanzim/system-design-primer)
11.2. [LeetCode posts on System Design](https://leetcode.com/discuss/interview-question/1140451/helpful-list-of-leetcode-posts-on-system-design-at-facebook-google-amazon-uber-microsoft)

12. Python - Pipenv

$ python3 -m pip install --upgrade pip
$ python3 -m pip install virtualenv
$ virtualenv --version
$ virtualenv project_workspace
$ cd project_workspace
$ source bin/activate
$ pip install requests
$ pip freeze > requirements.txt
$ deactivate
$ pip install -r requirements.txt

13. Python Virtual Environments

$ python3 -m venv env
$ source env/bin/activate
$ pip install --upgrade pip
$ pip install django
$ pip install djangorestframework
$ pip freeze > requirements.txt
$ pip install -r requirements.txt
$ deactivate

14. SQL Group by time

Hourly Group By

SELECT
    restaurant,
    DATE_TRUNC('hour', payment_time) AT TIME ZONE 'America/New_York' AS payment_datetime,
    COUNT(id) AS total
FROM
    payments
WHERE payment_time AT TIME ZONE 'America/New_York' BETWEEN '2023-04-03 00:00:00' AND '2023-04-03 23:59:59'
AND restaurant IN ('tim_hortons','mcdonalds','kfc')
GROUP BY restaurant, payment_datetime;

15 Minutes Group By

SELECT
    restaurant,
    TO_TIMESTAMP(FLOOR((EXTRACT('epoch' FROM payment_time) / 900 )) * 900) AT TIME ZONE 'America/New_York' AS payment_datetime,
    COUNT(id) AS total
FROM
    payments
WHERE payment_time AT TIME ZONE 'America/New_York' BETWEEN '2023-04-03 00:00:00' AND '2023-04-03 23:59:59'
AND restaurant IN ('tim_hortons','mcdonalds','kfc')
GROUP BY restaurant, payment_datetime;```