DSA topic · Stage 1

Strings interview preparation

Treat text as structured sequences with encoding, frequency, and substring constraints.

DSA prep Practice 10 questions

Concept

String questions often test careful indexing and a pattern such as a window, trie, or frequency map.

Complexity lens

Account for repeated slicing, immutable string construction, and alphabet-size assumptions.

Common mistakes

Assuming ASCII without saying so
Creating quadratic substrings
Skipping normalization requirements

Recognition

Decide from constraints, not labels.

These clues narrow the model. They do not replace an invariant or complexity check.

Recognition clues

The answer depends on a substring, prefix, suffix, or character counts
Normalization or encoding rules affect equality
Repeated construction may hide copying cost

Implementation options

Index scan over code units under a stated character model
Frequency map or fixed alphabet table
Two pointers, sliding window, trie, or builder depending on the invariant

Interview behavior

Clarify case, whitespace, punctuation, and character set
Avoid claiming O(1) alphabet space without an assumption
Narrate how the chosen representation preserves the required text semantics

Patterns that operate on this topic

Frequency MapThe table contains exactly the information from the processed prefix that a future element may need.Two PointersEvery candidate outside the active pointer region has already been resolved or proved unable to improve the answer.Sliding WindowAfter the repair loop, the active window satisfies the declared condition and its maintained state equals exactly its contents.StackThe stack contains unresolved items from the processed prefix in exactly the order they may be closed or dominated.TrieA path from root spells a prefix present in inserted keys; a terminal marker distinguishes a complete key from a prefix only.

Representative problems

10 current Strings questions.

Exact active distribution: 5 easy, 5 medium, 0 hard. External records contain metadata and original guidance only.

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Valid Anagram
Practice frequency accounting and character-set assumptions.
Strings Hash Maps Frequency Map
EasyFree / publicSolve problem
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Group Anagrams
Design a stable grouping key and explain its cost.
Strings Hash Maps Sorting Frequency Map
MediumFree / publicSolve problem
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Valid Palindrome
Clarify normalization rules before moving inward from both ends.
Strings Two Pointers
EasyFree / publicSolve problem
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Longest Substring Without Repeating Characters
Maintain the smallest valid left boundary as characters repeat.
Strings Hash Maps Sliding Window
MediumFree / publicSolve problem
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Valid Parentheses
Store unresolved opening delimiters and reject mismatches immediately.
Strings Stacks & Queues Stack
EasyFree / publicSolve problem
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Implement Trie (Prefix Tree)
Distinguish a complete word from a reachable prefix.
Tries Strings Trees Trie
MediumFree / publicSolve problem
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Isomorphic Strings
Tests whether one-way mapping logic misses collisions.
Arrays Strings Hash Maps Frequency Map
EasyFree / publicSolve problem
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Maximum Average Subarray I
Separates fixed-window bookkeeping from variable-window logic.
Arrays Strings Sliding Window
EasyFree / publicSolve problem
External catalogUnverified
Permutation in String
Tests whether counts can be updated without rebuilding each window.
Arrays Strings Sliding Window
MediumFree / publicSolve problem
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Longest Repeating Character Replacement
Builds a subtler validity condition and follow-up reasoning.
Arrays Strings Sliding Window
MediumFree / publicSolve problem

Review state

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Review prompts

What character-set assumption did I make?
Where did I allocate substrings?
Could a moving boundary preserve the same information?

Prerequisites and next step

Core Patterns

Learn the reusable movement and state patterns that remove brute-force work.

Open the roadmap

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