c++ 使用unordered_set实现hashset

使用unordered_set库来构造hashset

unordered_set 官方文档介绍

Unordered sets are containers that store unique elements in no particular order, and which allow for fast retrieval of individual elements based on their value.
In an unordered_set, the value of an element is at the same time its key, that identifies it uniquely. Keys are immutable, therefore, the elements in an unordered_set cannot be modified once in the container - they can be inserted and removed, though.
Internally, the elements in the unordered_set are not sorted in any particular order, but organized into buckets depending on their hash values to allow for fast access to individual elements directly by their values (with a constant average time complexity on average).
unordered_set containers are faster than set containers to access individual elements by their key, although they are generally less efficient for range iteration through a subset of their elements.
Iterators in the container are at least forward iterators.
简单来说，unordered_key非常的高效，使用值本身作为键值

Example

#include<iostream>
#include<unordered_set>
using namespace std;

int main() {
    // 1. initialize a hash set
    unordered_set<int> hashset;
    // 2. insert a new key
    hashset.insert(1);
    hashset.insert(2);
    hashset.insert(3);
    // 3. delete a key
    hashset.erase(2);
    // 4. check if the key is in the hash set
    if (hashset.count(2) <= 0) {
        cout << "Key 2 is not in the hash set." << endl;
    }
    // 5. get the size of the hash set
    cout << "The size of the hash set is " << hashset.size() << endl;
    // 6. iterate the hash set
    for (auto it = hashset.begin(); it != hashset.end(); it++) {
        cout << (*it) << " ";
    }
    cout << "is in the hash set." << endl;
    // 7. clear the hash set
    hashset.clear();
    // 8. check if the hash set is empty
    if (hashset.empty()) {
        cout << "hash set is empty now!" << endl;
    }
    
    return 0;
}