Stacks and queues are restricted sequences: they expose only the ends, and that restriction is exactly what makes them useful. A stack is LIFO (last-in, first-out) — push/pop/peek at one end. A queue is FIFO (first-in, first-out) — enqueue at the back, dequeue at the front. Both give O(1) operations and model the two most common access patterns in interviews: "undo the most recent thing" and "process in arrival order."
Where they show up
Stacks back the call stack, expression parsing, DFS, and backtracking. Queues back BFS, scheduling, and producer/consumer buffers. A deque (double-ended queue) generalizes both — push/pop at either end — so one structure covers stack and queue.
Monotonic stacks
A monotonic stack keeps its elements sorted (increasing or decreasing) as you scan. Before pushing, you pop everything that violates the order. Each element is pushed and popped at most once, so a scan that looks nested is actually O(n). This is the trick behind Next Greater Element, Largest Rectangle in Histogram, and stock-span problems — any "find the nearest larger/smaller neighbor" question.
Circular queues
A fixed-size circular buffer (ring buffer) wraps a head and tail index around an array with modular arithmetic, giving O(1) enqueue/dequeue without shifting. It is the standard bounded-queue implementation — ArrayBlockingQueue works this way.
Java mapping
The coding problems below — valid parentheses, min-stack, queue-from-stacks, and the monotonic-stack family — are where these patterns become reflexes.