The string deduplication problem is a canonical one within computer science, serving a similar purpose as fizz-buzz in terms of being an example of a simple problem that a reasonably knowledgable practitioner should be able to solve with minimal effort.
The problem appears in a few variants, but briefly one such variant is to remove duplicate letters in a given string, such that the string then has only one instance of any given letter.

The shunting yard algorithm converts infix expressions (i.e. 1+2) into reverse Polish notation, i.e. 1 2 +, which lends itself well to execution on a stack machine.
An aside: I wanted to revisit this algorithm because it was one of the first I implemented in C during self-study five years ago. In a way, reimplementing it is a way of measuring my progress since then.
The internal details aren’t too complicated - it’s based on the simple pseudo-code of the Wikipedia article describing the shunting yard algorithm.

The Pythagorean triple problem is as follows. Given an input integer \(n\), return integers \(a\), \(b\), \(c\) such that the two following conditions hold:
$$ a b c = n $$ $$ a^2 + b^2 = c^2 $$
I was interested in finding a solution to this problem that was both succint and had good asymptotic complexity. The solution I found runs in O(sqrt(n)) time by deconstructing the problem into the well-known 3SUM problem.