Int implementation using linked lists

This is the first piece of code I've written on my own since I started learning a couple of months ago. The code works and it takes an int as input and creates a linked list with each node representing a digit. I've added a function that sums two linked listed ints and returns a third list with the result. The rest of the code is also distributed in functions.

Before I move on, it would be greatly appreciated to know if I'm doing something wrong, if my code is properly structured or if I'm following a good/bad style.

#include <iostream>

using std::cout;
using std::cin;

struct intNode {
    int num;
    intNode *next;
};

int deallocateList(intNode *&s);
int intConversionHelper(int n);
int lenList(intNode *&s);
int sumListContent(intNode *&s);
intNode *sumLists(intNode *&s1, intNode *&s2);
int printNode(intNode *&s);
int intLen(int n);
intNode *intToList(int n);
void append(intNode *&s, int n);



int main() {
    int num1;
    int num2;
    cout << "Give me number 1: ";
    cin >> num1;

    cout << "Give me number 2: ";
    cin >> num2;

    intNode *list1 = intToList(num1);
    intNode *list2 = intToList(num2);
    cout << "\nPrinting ints as linked lists: \n";
    printNode(list1);
    printNode(list2);

    cout << "\nSum of the two lists (as a list too!)\n";
    intNode *sumList = sumLists(list1, list2);
    printNode(sumList);

    deallocateList(list1);
    deallocateList(list2);
    deallocateList(sumList);
}

int printNode(intNode *&s) {
    intNode *tmp = s;

    while (tmp != NULL) {
        cout << tmp->num;
        tmp = tmp->next;
    }
    cout << "\n";
}

int intLen(int n) {
    int len = 1;
    while (n > 9) {
        len++;
        n /= 10;
    }
    return len;
}

intNode *intToList(int n) {
    intNode *intList = NULL;
    int num;
    int dec = 10;

    num = n % 10;
    append(intList, num);
    int count = 1;
    while (count < intLen(n)) {
        num = n / dec % 10; 
        dec = dec * 10;
        append(intList, num);
        count++;
    }
    return intList;     
}

void append(intNode *&s, int n) {
    if (s == NULL) {
        s = new intNode;
        s->num = n;
        s->next = NULL;
    }
    else {
        intNode *nNode = new intNode;
        nNode->num = n;
        nNode->next = s;
        s = nNode;
    }
}

intNode *sumLists(intNode *&s1, intNode *&s2) {
    int sum1 = sumListContent(s1);
    int sum2 = sumListContent(s2);
    return intToList(sum1 + sum2);
}

int sumListContent(intNode *&s) {
    intNode *tmp = s;
    int sum = 0;
    int len = lenList(s);
    while (tmp != NULL) {
        sum += intConversionHelper(len) * tmp->num;
        len--;
        tmp = tmp->next;
    }
    return sum;
}

int lenList(intNode *&s) {
    intNode *tmp = s;
    int len = 0;
    while (tmp != NULL) {
        len++;
        tmp = tmp->next;
    }
    return len;
}

int intConversionHelper(int n) {
    float value = n;
    for (int i = 1; i < n; i++) {
    value = value * 10;
    }
    value = value / n;
    return value;
}

int deallocateList(intNode *&s) {
    intNode *tmp = s;
    while (s != NULL) {
        tmp = tmp->next;
        delete[] s;
        s = tmp;
    }
}