Linked List
Pointer rewiring — reverse, merge, middle-via-fast-slow, cycle detection (Floyd's).
Linked list problems aren't a single algorithm — they're a small toolbox of canonical pointer-rewiring techniques. Once you know all four (reverse, merge, fast-slow, cycle), you can compose them to solve essentially any linked-list problem.
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Monotonic Stack / Deque
Maintain a stack or deque whose elements stay monotonic — pop violators on every push. The unlock for "next greater" and "sliding window max" problems.
Linked List Manipulation
Pointer rewiring on singly linked lists — reverse, merge, find middle (fast/slow), detect cycle (Floyd's), and the four ops that compose to solve almost any LL problem.