DSA · INTERVIEW & OA PREP

Topic 1 of 12

Arrays & Strings

The most common OA and first-round topic — most warm-up problems are array or string manipulation. Master in-place traversal, prefix sums, and sliding windows here and a large share of easy/medium questions become routine.

Key concepts

• Two-pointer technique for pairs, reversals, and partitioning in O(n).
• Sliding window for subarray/substring problems (max sum, longest without repeats).
• Prefix sums to answer range-sum queries in O(1) after O(n) setup.
• Strings are arrays of characters — watch for immutability in Java/Python.

Questions that get asked

• Reverse a string / array in place.
• Two-sum: find a pair that adds to a target.
• Longest substring without repeating characters.
• Maximum subarray sum (Kadane's algorithm).
• Move all zeros to the end while keeping order.