Y1S2 Sort Linked List in Ascending Order Homework

#include <iostream>
using namespace std;

struct elem {
    int key;
    struct elem* next;
} *start;

void init() {
    start = NULL;
}

struct elem* swap(struct elem* p, struct elem* q) {
    struct elem* tmp = q->next;
    q->next = p;
    p->next = tmp;
    return q;
}

int sort(struct elem** start, int list_size) {
    struct elem** s;
    int i, j, swapped = 0;
    for (i = 0; i <= list_size; i++) {
        s = start;
        for (j = 0; j < list_size - i - 1; j++) {
            struct elem* p = *s;
            struct elem* q = p->next;
            if (p->key < q->key) {
                *s = swap(p, q);
                swapped = 1;
            }
            s = &(*s)->next;
        }
        if (swapped == 0)
            break;
    }
    return 0;
}

void add_b(int n) {
    elem* p = start;
    start = new elem;
    start->key = n;
    start->next = p;
}

void add_e(int n) {
    elem* p = start, * q = start;
    q = new elem;
    q->key = n;
    q->next = NULL;
    if (start) {
        while (p->next)
            p = p->next;
        p->next = q;
    }
    else
        start = q;
}

void output(struct elem* n) {
    while (n != NULL) {
        cout << n->key << "  |  ";
        n = n->next;
    }
    cout << endl;
}

int main() {
    init();
    int n, x, i = 0;

    cout << " > Enter key value for start element |  ";
    cin >> n;
    add_b(n);
    i++;

    cout << " > Enter how many elements to add after start |  ";
    cin >> x;
    for (int j = 0; j < x; j++) {
        cout << " > Enter key value for element " << i << " |  ";
        cin >> n;
        add_e(n);
        i++;
    }

    cout << " > List before sort |  ";
    output(start);

    sort(&start, i);

    cout << " > List after sort |  ";
    output(start);

    return 0;
}