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

struct node {
 int val;
 node *next;
};

void add(node*& H, int x ) {
    node * p = new node;
    p->val = x;
    p->next = H;
    H=p;
}

int size(node* H) {
    int size = 0;
    while(H != NULL) {
        H = H->next;
        size++;
    }
   return size;
}

void show(node* H) {
    node * p = H;
    cout <<"HEAD->";
    while(p != NULL) {
        cout << p->val <<"->";
        p = p->next;
    }
    cout << "NULL"<<endl;
}

void delTop(node*& H) {
    if( H != NULL) {
        node *p = H;
        H = p->next;
        delete p;
    }
}

void delX(node*& H, int x) {
    if(H != NULL) {
        node *p = H;
        if(H->val == x) {
            delTop(H);
        } else {
            while(p->next != NULL && p->next->val != x) {
                p = p->next;
            }

            if(p->next != NULL) {
                delTop(p->next);
            }
        }
    }
}

void delEverySecondElement(node*& H) {
    delTop(H);
    node *p = H;
    node *p2 = H;
    while(p != NULL && p->next != NULL) {
        p = p->next;
        p2->next = p->next;
        p2 = p2->next;
        delete p;
        p = p2;
    }

}

void copyNumberBehindIfisEven(node*& H) {
    node *p = H;
    node *p2 = H;
    while(p != NULL && p->next != NULL) {
        if(p->val%2 == 0) {
            cout << p->val;
        }
        p = p->next;
    }

}


bool isEmpty(node* H) {
    if(H !=NULL) {
        return 0;
    } else {
        return 1;
    }
}

void top(node* H) {
    if(H!=NULL) {
        cout << H->val << endl;
    } else {
        cout << "empty";
    }
}

void push() {}

void pop() {}

int main()
{
    node *H = NULL;
    add(H, 12);
    add(H, 17);
    add(H, 18);
    add(H, 20);
    add(H, 22);
    add(H, 24);
    add(H, 2);
    add(H, 4);
    add(H, 6);
    show(H);
    cout << "Rozmiar: " << size(H) << endl;
    delEverySecondElement(H);
    show(H);

    copyNumberBehindIfisEven(H);

    return 0;
}