DSA is all about patterns
know the pattern, know the code.

The art of O(1) lookup — every hard problem becomes easy

Two moves, one pass — cuts O(n²) to O(n) every time

The secret behind 90% of subarray questions at FAANG

Pre-compute once, answer range queries in O(1) forever

If you can ask 'is X possible?', you can binary search

Sort first — most greedy and two-pointer tricks need it

Make the locally optimal choice — prove it works globally

LIFO magic — monotonic stack solves histogram, spans, more

BFS lives here — deque unlocks O(n) sliding window max

Fast & slow pointers — detect cycles, find midpoints

Build the decision tree — prune early, get all answers

DFS, BFS, LCA — master these 4 traversals, own the tree

Ordered structure unlocks O(log n) search, insert, delete

Top-K in O(n log k) — indispensable for streaming data

Sort by start, sweep line — merge/insert intervals clean

Prefix tree for autocomplete, word search — must-know

BFS for shortest path, DFS for connectivity — graph mastery

Optimal substructure + overlapping subproblems = DP

XOR, shifts, masks — bit tricks that stun interviewers

GCD, primes, modular arithmetic — pure math in interviews

Split, solve, merge — the idea behind MergeSort and FFT

Scale to millions — design patterns every senior knows