Code : Concatenated Multiples

#include <bits/stdc++.h>
using namespace std;

int get_len(int num) { return to_string(num).length(); }

void solve(vector<long long> &a, int k) {

    int n = a.size();
    vector<long long> pow10(10 + 1);
    pow10[0] = 1;
    for (int i = 1; i <= 10; i++) {
        pow10[i] = (pow10[i - 1] * 10) % k;
    }

    map<int, map<int, int>> md;
    for (int i = 0; i < n; i++) {
        md[get_len(a[i])][a[i] % k]++;
    }

    long long ans = 0;
    for (int i = 0; i < n; i++) {
        // To avoid counting (i == j), temporarily remove this element.
        md[get_len(a[i])][a[i] % k]--;

        for (int len = 1; len <= 10; len++) {
            int have = (pow10[len] * (a[i] % k)) % k;
            int need = (-have) % k;
            if (need < 0) {
                need += k;
            }
            ans += md[len][need];
        }

        // Add the element back.
        md[get_len(a[i])][a[i] % k]++;
    }
    cout << ans << "\n";
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int n, k;
    cin >> n >> k;
    vector<long long> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    solve(a, k);
    return 0;
}

#include <bits/stdc++.h>
using namespace std;

void solve(vector<long long> &a, int k) {
    int n = a.size();
    vector<int> getlen(n);
    vector<long long> getmod(n);

    vector<long long> pow10(10 + 1);
    pow10[0] = 1;
    for (int i = 1; i <= 10; i++) {
        pow10[i] = (pow10[i - 1] * 10) % k;
    }

    map<int, map<int, int>> md;
    for (int i = 0; i < n; i++) {
        getlen[i] = to_string(a[i]).length();
        getmod[i] = a[i] % k;
        md[getlen[i]][getmod[i]]++;
    }

    long long ans = 0;
    for (int i = 0; i < n; i++) {
        // To avoid counting (i == j), temporarily remove this element.
        md[getlen[i]][getmod[i]]--;

        for (int len = 1; len <= 10; len++) {
            int have = (pow10[len] * (getmod[i])) % k;
            int need = (-have) % k;
            if (need < 0) {
                need += k;
            }
            auto &mp = md[len];
            if (mp.find(need) != mp.end()) {
                ans += mp[need];
            }
        }

        // Add the element back.
        md[getlen[i]][getmod[i]]++;
    }
    cout << ans << "\n";
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int n, k;
    cin >> n >> k;
    vector<long long> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    solve(a, k);
    return 0;
}