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Queue

easy

Two indices, neither ever moving backwards. It looks trivial until you implement it on an array and discover that shifting on every dequeue makes a full drain O(n²).

Worst O(1) enqueue/dequeueSpace O(n)
Loading the queue engine…

Where to go next

Circular queue (ring buffer)

medium

Modulo turns a fixed array into an endless stream.

Deque

medium

A queue open at both ends.

Breadth-first search

easy

Expands in rings, so it finds shortest paths.