巧妙收缩 vector 空间

resize() 或者 erase() 后 vector 的空间大小不会自动减小

int main()
{
    vector<int> v;
    for(int i = 0; i < 100000; ++i){
        v.push_back(i);
    }
    cout << "size: " << v.size() << endl;
    cout << "capacity: " << v.capacity() << endl;

    cout << "-----------------------" << endl;
    v.resize(10);
    cout << "size: " << v.size() << endl;
    cout << "capacity: " << v.capacity() << endl;
    
    cout << "-----------------------" << endl;
    vector<int>(v).swap(v);
    cout << "size: " << v.size() << endl;
    cout << "capacity: " << v.capacity() << endl;
    
    return 0;
}

输出：

size: 100000
capacity: 131072
-----------------------
size: 10
capacity: 131072
-----------------------
size: 10
capacity: 10