C++ std::unordered_map & unordered_set: Hash Tables, Buckets & O(1) Operations

std::unordered_map and std::unordered_set utilize Hash Tables to achieve average $O(1)$ Constant-Time insertion, lookup, and deletion.

Benchmarking: When to choose `std::unordered_map`

Container	Underlying Structure	Average Lookup	Worst-Case Lookup	Ordering
`std::map`	Red-Black Tree	$O(log N)$	$O(log N)$ Guaranteed	Sorted
`std::unordered_map`	Hash Table	$O(1)$ Constant	$O(N)$ (High collisions)	Unordered / Arbitrary

Frequently Asked Questions (FAQ) - Student & Interview Review

Q1. What is the difference between std::map and std::unordered_map?

std::map is implemented as a Red-Black Tree, maintaining keys in sorted order with guaranteed O(log N) lookups. std::unordered_map is implemented as a Hash Table, providing faster average O(1) lookups, but keys are unsorted.

Q2. What is a Hash Collision in std::unordered_map?

A collision occurs when two distinct keys produce the exact same hash code. C++ resolves collisions using Separate Chaining (each bucket contains a linked list of entries that share the hash).