Patterns

signal O(1) lookups, dedupe, or counting frequencies

signal sorted array, pair/triplet, or converging from both ends

signal longest / shortest / best contiguous subarray or substring

signal matching pairs, undo, or "next greater" (monotonic stack)

signal sorted input — or "minimise the max / search on the answer"

signal reorder/reverse in place, or fast–slow pointers for cycle/middle

signal recurse on subtrees — return-up vs pass-down; BFS for levels

signal grid or adjacency; components; unweighted shortest path → BFS

signal count ways / best value with overlapping subproblems along a line

signal top-K, running median, or repeatedly grab the min/max

signal generate all subsets/combos/permutations — choose·explore·unchoose

signal overlapping ranges → sort by start/end, then sweep

signal local best choice provably gives the global best; one pass

signal two sequences or a grid; state depends on two indices

signal prefix search, autocomplete, dictionary of words

signal ordering with dependencies (topo), or union-find to merge sets

signal XOR tricks, counting bits, no-extra-space via bits

signal in-place matrix moves, simulation, number tricks