A stack that knows its minimum in Java
Java interview exercise on Generics & Optional, difficulty 3 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write class MinStack<T extends Comparable<T>> with push(v), pop() (returns the removed value), peek(), getMin() and isEmpty(), where getMin() answers in constant time: keep a second stack of the minimums seen so far. pop, peek and getMin on an empty stack throw NoSuchElementException.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.