If you’ve been grinding DSA problems, you’ve probably noticed a pattern —
90% of interview questions start with just three things: Arrays, Hashmaps, or Strings.
Why? Because these are the building blocks. If you can bend these data structures to your will, you can probably solve anything an interviewer throws at you.
Let’s break it down.
🔹 Arrays – The Playground of Patterns
Arrays are where most companies start. Why?
- They’re simple.
- Easy to visualize.
- And they force you to think in indexes, boundaries, and iterations.
Classic problems:
- Two Sum → Brute force vs Hashmap optimization.
- Maximum Subarray (Kadane’s Algorithm) → Teaches you DP intuition.
- Sliding Window on Arrays → An entire class of problems (longest substring, max sum subarray of size k, etc).
👉 If you master arrays, you’ll automatically start spotting patterns in other problems.
🔹 Hashmaps – The Secret Weapon
Whenever you feel stuck, 70% of the time the hidden trick is:
“Use a hashmap.”
Why interviewers love them:
- They test if you know time complexity trade-offs.
- They force you to think about hashing & collisions.
- They’re insanely practical (real systems use them everywhere).
Examples:
- Counting frequencies (Anagrams, Majority Element, Word Count).
- Tracking indexes (Two Sum, Subarray with Sum k).
- Caching results (Dynamic Programming memoization).
Hashmaps basically test whether you can store and retrieve information smartly instead of brute forcing.
🔹 Strings – Where Arrays & Hashmaps Collide
Strings are just arrays of characters. But they’re powerful because they combine both arrays & hashmaps.
Typical patterns:
- Substring problems → Sliding window + Hashmap (Longest substring without repeating characters).
- Anagrams & Palindromes → Character counts with hashmaps.
- String Matching → Think brute force vs optimized (KMP, Rabin-Karp).
Why interviewers love them:
- Strings bring real-world flavor (search, text, data parsing).
- They can test multiple concepts at once: arrays + hashing + logic.
🎯 Why These Three Dominate Interviews
- Simplicity → Depth
They’re simple to explain, but solutions can go from O(n²) brute force → O(n) optimized. Perfect for interviews. - Transferable Patterns
Once you understand sliding windows, prefix sums, hashmaps, etc., you can solve harder graph/DP problems faster. - Universal Filter
Doesn’t matter if you’re from a startup, freelancing, or FAANG — everyone should be able to handle these basics.
🏆 Takeaway
Before diving deep into trees, graphs, and advanced DP, make sure you’re unbeatable in arrays, hashmaps, and strings.
Because in interviews, these aren’t just data structures.
👉 They’re the language of problem-solving.
✍️ Personal Note
When I started prepping, I underestimated these basics. I thought, “Arrays and Strings? That’s beginner stuff.” But the deeper I went, the more I realized — every complex problem has roots in these simple structures.
So now, I treat arrays, hashmaps, and strings like gym warm-ups.
Get strong in them → everything else becomes easier.