Strong candidates don't know more algorithms — they recognise problems faster. Interview problems are variations of ~15 reusable patterns. Learn to spot the signal words, and half the battle is over before you write code.
The signal → pattern table
The problem says... → Reach for... ───────────────────────────────────────────────────────────── "sorted array" + find/search → Binary Search "pair / triplet that sums to..." → Two Pointers (sorted) or HashMap "longest/shortest SUBSTRING or SUBARRAY" → Sliding Window "linked list" + cycle / middle → Fast & Slow Pointers "overlapping intervals / meetings" → Merge Intervals "top K / K largest / K most frequent" → Heap (or sort if small) "how many ways / min cost / max value" → Dynamic Programming "all combinations / permutations" → Backtracking (DFS) "shortest path / level by level" → BFS "valid parentheses / undo / nearest greater" → Stack "range sum queries" → Prefix Sum "count / frequency / seen before?" → HashMap / Set
The 4-step routine for every problem
- Restate + clarify — inputs, outputs, edge cases ("empty array? duplicates? negatives?").
- Say the brute force out loud — "nested loops give O(n²)". This scores points AND anchors your optimisation.
- Scan the signal table — sorted? substring? K? intervals? Pick the pattern candidate.
- Apply the pattern template — each lesson in this course gives you one to adapt.
How to use this course
Each lesson = one pattern: when to use it, the code template, a worked example, and problems on our DSA judge to solve immediately. Don't just read — the pattern only sticks after you've used it twice. Start with Two Pointers.