unordered multiset method in C++

* create
```
#include <unordered_set>
using namespace std;

int main () {
    unordered_multiset<int> a;  // default constructor
    unordered_multiset<int> b(a);  // copy constructor
    unordered_multiset<int> c(move(a));  // move constructor
    unordered_multiset<int> d{1,2,3};  // initializer list constructor
    // unordered_multiset has no fill constructor
    unordered_multiset<int> f(a.begin(),a.end());  // range constructor

    unordered_multiset<int>* g = new unordered_multiset<int>{1,2,3};  // new operator
    delete g;
}
```
* iterator
```
#include <iostream>
#include <unordered_set>
using namespace std;

int main (){
  unordered_multiset<int> s{1,2,3,4,5,1};

  for (auto it=s.begin(); it!=s.end(); ++it)
    cout<<*it<<",";
  cout<<endl;
  for (auto it=s.cbegin(); it!=s.cend(); ++it)
    cout<<*it<<",";
  cout<<endl;
}
/*
5,4,3,2,1,1,
5,4,3,2,1,1,
*/
```
* add
```
#include <iostream>
#include <unordered_set>
using namespace std;

int main ()
{
    unordered_multiset<int> m;

    m.insert(1);
    m.emplace(1);

    for (auto &i:m)
        cout<<i<<";";
    cout<<endl;
}
// 1;1;
```
* change
```
#include <iostream>
#include <unordered_set>
using namespace std;

void p(unordered_set<int>& a) {
    for(auto &i:a) cout<<i<<";";
    cout<<endl;
}

int main ()
{
    unordered_set<int> a{1,2,3,4,5};
    unordered_set<int> a1(a), a2(a);

    a.swap(a1);
    p(a);
}
```
* query
```
#include <iostream>
#include <unordered_set>
using namespace std;

int main() {
    unordered_multiset<int> s{1,2,3,4,5,6,1,2,3};

    auto it=s.find(2);
    if(it!=s.end()) cout<<"found"<<endl;
    cout<<s.count(2)<<endl;

    auto r=s.equal_range(3);
    for(auto it=r.first; it!=r.second; it++)
        cout<<*it<<",";
    cout<<endl;

    cout<<s.empty()<<endl;
    cout<<s.size()<<endl;
    cout<<s.max_size()<<endl;
}
/*
found
2
3,3,
0
9
576460752303423487
*/
```
* delete
```
#include <unordered_set>
using namespace std;

int main () {
    unordered_multiset<int> s{1,2,3,4,5,1,2,3};

    s.erase(s.begin());
    s.erase(4);
    s.clear();
}
```
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