DSA & Coding (Interview-Grade)

The goal isn’t “know all algorithms”. The goal is to reliably:

• identify the right approach quickly
• implement correctly under time
• communicate tradeoffs

Core patterns you must recognize

Arrays / strings

• Two pointers
• Sliding window
• Prefix sums / difference arrays
• Sorting + scanning

Hashing

• Frequency maps
• De-dup and membership
• “Seen set” to break cycles

Trees / graphs

• BFS vs DFS decision rules
• Topological ordering (DAG)
• Shortest path when edges have weights

Dynamic programming

• Define state + transition + base case
• Start with 2D DP, then optimize
• If stuck: recursion + memo first

How interviewers evaluate you

They’re typically scoring:

• correctness and edge cases
• complexity reasoning
• clarity (naming, structure)
• debugging ability

A 45-minute coding process

1. Restate problem and confirm inputs/outputs
2. Identify constraints and edge cases
3. Propose 1–2 approaches; pick one
4. Write code in small compilable chunks
5. Test with 2–3 cases (happy path + edge)
6. State complexity and any follow-ups

Common failure cases

• Off-by-one bugs in loops/pointers

  • Fix: write invariants (what’s true each iteration)

• Wrong data structure (e.g., list instead of set)

  • Fix: ask “do I need membership in O(1)?”

• DP panic

  • Fix: first write recursion with memo; only then convert to bottom-up

Practice plan (minimum effective dose)

• 3 days/week: 1 medium problem timed (45–60 min)
• 2 days/week: 30 min pattern drill (same pattern, different problems)
• 1 day/week: redo a previously-missed problem from scratch

Notes by company archetype

• FAANG: speed + clear communication; they expect structured thinking.
• Enterprise: still cares about correctness, but may value maintainability and tests more.
• Automotive/industrial: correctness and disciplined reasoning can outweigh “flashy” speed.