A simple Task (CF) (Simple cycle counting)

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

int N, M;
int g[20][20];
vector<vector<long long>>dp(20, vector<long long>(1<<20, -1));
long long solve(int node, int mask) {
    long long &ret = dp[node][mask];
    if(ret!=-1) return ret;
    ret = 0;
    int smallest = __builtin_ctz(mask);
    if(__builtin_popcount(mask)>2) ret += g[node][smallest];
    for(int to=smallest+1; to<N; ++to) if(~mask >> to & 1) {
        if(g[node][to]) ret += solve(to, mask|(1<<to));
    }
    return ret;
}
void PlayGround() {
    cin >> N >> M;
    for(int i=0; i<M; ++i) {
        int u, v; cin >> u >> v;
        --u, --v;
        g[u][v] = g[v][u] = 1;
    }
    long long ans = 0;
    for(int i=0; i<N; ++i) {
        ans += solve(i, (1<<i));
    }
    cout << ans/2 << '\n';


    // cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
}
int main() {

    PlayGround();
    return 0;
}