[CSES] Game Routes

void solve() {
    ll n, m;
    cin >> n >> m;

    vvll v(n);
    rep(i, m) {
        ll x, y;
        cin >> x >> y;
        --x, --y;
        v[x].pb(y);
    }

    vll topo, vis(n, 0);
    function <void(ll, ll)> dfs = [&](ll node, ll pr) {
        vis[node] = 1;
        for (auto& child : v[node]) {
            if (!vis[child]) dfs(child, node);
        }
        topo.pb(node);
        };
    dfs(0, 0);
    reverse(all(topo));

    vll dp(n, 0);
    dp[0] = 1;
    for (auto& node : topo) {
        for (auto& child : v[node]) {
            dp[child] += dp[node];
            dp[child] %= mod;
        }
    }

    cout << dp[n - 1] << "\n";
}