All notes, in reading order

Kth Largest Element (Sorting Approach)

Kth Largest Element (LeetCode 215) via sorting: sort and index from the end in O(n log n), with a note on the O(n)-average quickselect alternative.

Relative Sort Array

Relative Sort Array (LeetCode 1122): sort by the order defined in a second array, with the rest sorted ascending. O(n log n), Python/Java/C++.

Wiggle Sort

Wiggle Sort (LeetCode 280): rearrange so nums[0] = nums[2]

Wiggle Sort II

Wiggle Sort II (LeetCode 324): strict nums[0] < nums[1] > nums[2]... via sorting and interleaving the two halves in reverse. O(n log n), Python/Java/C...

H-Index (Sorting Approach)

H-Index (LeetCode 274) via sorting: sort citations descending and find the largest i where citations[i] >= i+1. O(n log n), Python/Java/C++.

Meeting Rooms (Can Attend All Meetings)

Meeting Rooms (LeetCode 252): sort meetings by start and check that none overlaps the previous one. O(n log n), Python/Java/C++.

Write a SQL query to find the second highest salary from an Employees table without using LIMIT or TOP.

Write a query to find all customers who placed orders in every month of 2025.

Write a query to identify duplicate rows in a Transactions table based on customer_id, amount, and date.

Write a query using GROUP BY and HAVING to find departments with more than 10 employees earning above the department average salary.

Write a query to return the top 3 selling products per category using a subquery (no window functions allowed).

Write a query to calculate the running total of daily revenue for a store using a self-join.