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Linked-list problems reward careful pointer bookkeeping over cleverness: reverse in place, find the middle with fast and slow pointers, detect a cycle, or merge two sorted lists. A dummy head node removes most of the edge cases people trip on. Draw the pointers before you write - almost every bug here is an off-by-one on the node you already moved past.
Any problem handing you a list head, especially with an O(1) space constraint.
12 problems. Each one runs in an in-browser editor against real test cases, with hints and a worked solution.
The patterns cheat sheet covers all of these in one page - what each one looks like in a problem statement, and the template that follows.