Hashing and maps
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Overview
A hash map stores data by key-value pairs. It gives near-constant time access on average.
Problems to practice
- First non-repeating character
- Valid anagram
- Top K frequent elements
- Group anagrams
- Two-sum using map
- Subarray sum equals k
- Longest substring with K distinct characters
Key idea
Use a map when you need fast lookup, counts, or grouping.
Time and space complexity
- Put: O(1) average
- Get: O(1) average
- Delete: O(1) average
- Space: O(n)
Adapted from 14_DSA_MASTER.md · “4. Hashing and maps”. Code examples are study fragments. Difficulty and core labels are editorial estimates; version-specific behavior should be checked against your project.