Hash Maps and Sets

Hash-based data structures like Hash Maps (key–value pairs) and Hash Sets (unique elements) are extremely powerful tools for solving problems efficiently.
They allow fast lookups, insertions, and deletions, usually in O(1) average time complexity.

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When to Use Hash Maps and Sets

Fast Membership Checking
Use a Hash Set when you need to quickly check if an element has been seen before.
Example: Detect duplicates in an array.

Counting Frequencies
Use a Hash Map to count occurrences of items (numbers, characters, words).
Example: Find the most frequent element in a list.

Mapping Relationships
Use a Hash Map when you need to associate keys with values.
Example: Build an index of words to their positions in a document.

Avoiding Nested Loops
Hash Maps and Sets help reduce time complexity by replacing brute-force searches with constant-time lookups.
Example: Check if two arrays share a common element in O(n) instead of O(n²).

Tracking State Across Iterations
They are useful to store partial results or states as you traverse a structure.
Example: Prefix sum problems where you map cumulative sums to indices.

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