# <unordered_set>

**[<unordered_set>](https://en.cppreference.com/w/cpp/header/unordered_set)** is a hash table of unique elements with O(1) average insertion, deletion, and lookup. Elements are unordered but quickly accessible.

Use `unordered_set` for fast membership testing; use `std::set` if you need sorted order.

## Example

This example detects duplicate values in a sequence by checking and inserting into an unordered_set, tracking first-seen items.

```cpp
// compile: g++ -std=c++11 -o unorderedsetexample unorderedsetexample.cpp
// run: ./unorderedsetexample
// description: hash set membership testing

#include <unordered_set>
#include <iostream>

int main() {
    std::unordered_set<int> seen;
    
    for (int x : {1, 2, 3, 2, 1}) {
        if (!seen.count(x)) {
            std::cout << "first time seeing " << x << "\n";
            seen.insert(x);
        } else {
            std::cout << "already saw " << x << "\n";
        }
    }
    
    return 0;
}
```