A least-recently-used cache in a dozen lines: a LinkedHashMap in access order that removes its eldest entry when it grows past the capacity.
Check that every bracket is closed in the right order by pushing openers on a Deque and matching each closer against the top.
Retry a flaky async operation, doubling the wait between attempts.
Count word frequency, ignoring case, and take the five most common.
Two classic warm-up problems: reverse a string in place without Array.Reverse, and test a palindrome while ignoring case and punctuation.
Count how often each character appears using Dictionary of char to int, then print the most frequent first.
A small program that shows why override is polymorphism and new is only hiding: the same object gives different answers through different variable types.
Find two numbers that add up to a target in O(n) by remembering each value's index and looking up the complement.
Group products by category and calculate the inventory value of each category with GroupBy and Sum.
The classic Nth-highest question in LINQ: take the distinct values, sort them descending, skip one, and handle the case where there is no second value.
Check that brackets are properly closed and nested using Stack of char: push openers, pop and match on closers.
An xUnit test class with two tests, each laid out as Arrange, Act, Assert. Add the xunit package to a test project to run it.