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Код

#include <iostream>
#include <set>

template<typename T>
struct node
{
    T data;
    struct node *next;
    node(T data) : data(data), next(nullptr) {}
};

template<typename T>
class list
{
private:
    node<T> *head;
public:
    list() : head(nullptr) {}
    void insert(T data);
    void remove(T data);
    void show(void);
    void remove_dups(void);
};

template<typename T>
void list<T>::insert(T data)
{
    node<T> *tail = new node<T>(data);
    if (head == nullptr)
    {
        head = tail;
        return;
    }
    node<T> *iter = head;
    while (iter->next)
    {
        iter = iter->next;
    }
    iter->next = tail;
}

template<typename T>
void list<T>::remove(T data)
{
    node<T> *iter = head;

    if (!iter) return;    

    if (iter->data == data)
    {
        head = head->next;
        delete iter;
        return;
    }

    while (iter->next)
    {
        if (iter->next->data == data)
        {
            node<T> *next = iter->next;
            iter->next = iter->next->next;
            delete next;
            return;
        }
        iter = iter->next;
    }
} 
        
template<typename T>
void list<T>::show(void)
{
    node<T> *iter = head;
    while (iter)
    {
        std::cout << iter->data << std::endl;
        iter = iter->next;
    }
}    

template<typename T>
void list<T>::remove_dups(void)
{
    std::set<T> visited;
    node<T> *iter = head;
    node<T> *temp = nullptr;

    while (iter->next)
    {
        if (iter->next->data == *visited.find(iter->next->data))
        {
            temp = iter->next;
            iter->next = iter->next->next;
            delete temp;
        }
        else
        {
            visited.insert(iter->next->data);
        } 
        iter = iter->next;
    }
}

int main()
{
    list<int> ls;

    ls.insert(1);
    ls.insert(2);
    ls.insert(3);
    ls.insert(2);
    ls.insert(1);
    ls.insert(1);
    ls.insert(3);

    ls.show();

    ls.remove_dups();
    ls.show();

}