Submission #1287706

#	Time	Username	Problem	Language	Result	Execution time	Memory
1287706		adaawf	Star Trek (CEOI20_startrek)	C++20	0 / 100	2 ms	712 KiB

startrek

#include <bits/stdc++.h>
using namespace std;
vector<int> g[100005];
int f[100005], ff[100005], b[100005], s[100005];
long long int res[100005], mod = 1e9 + 7;
void dfs(int x, int p) {
    f[x] = 0; b[x] = p;
    s[x] = 1;
    for (int w : g[x]) {
        if (w == p) continue;
        dfs(w, x);
        s[x] += s[w];
        if (f[w] == 0) f[x] = 1;
    }
}
void dfs2(int x, int p) {
    int c = 0;
    for (int w : g[x]) {
        if (w == p) continue;
        c += 1 - f[w];
    }
    c += 1 - ff[x];
    for (int w : g[x]) {
        if (w == p) continue;
        c -= 1 - f[w];
        if (c) ff[w] = 1;
        c += 1 - f[w];
        dfs2(w, x);
    }
}
void dfs3(int x, int p, int flag, int z) {
    if (flag == 0) {
        for (int w : g[x]) {
            if (w == p) continue;
            dfs3(w, x, 1, z);
        }
        res[z]++;
    }
    else {
        if (g[x].size() > 2) return;
        for (int w : g[x]) {
            if (w == p) continue;
            dfs3(w, x, 0, z);
        }
    }
}
int main() {
    ios::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    long long int n, d;
    cin >> n >> d;
    for (int i = 1; i < n; i++) {
        int u, v;
        cin >> u >> v;
        g[u].push_back(v);
        g[v].push_back(u);
    }
    dfs(1, -1);
    ff[1] = 1;
    dfs2(1, -1);
    long long int c = 0, h = 0, k = 0, x, y, cc = 1;
    for (int i = 1; i <= n; i++) {
        int flag = 0;
        for (int w : g[i]) {
            if (w == b[i]) continue;
            if (f[w] == 0) flag++;
        }
        if (ff[i] == 0) flag++;
        c += flag;
        if (flag >= 2) {
            k += n;
        }
        else if (flag == 1) {
            for (int w : g[i]) {
                if (w == b[i]) continue;
                if (f[w] == 0) {
                    dfs3(w, i, 1, i);
                    k += n - s[w];
                }
            }
            if (ff[i] == 0) {
                dfs3(b[i], i, 1, i);
                k += n - s[i];
            }
            h += res[i];
        }
        else {
            dfs3(i, -1, 0, i);
            h += res[i];
        }
        if (i == 1) x = h, y = k;
    }
    h %= mod; k %= mod;
    //cout << c << " " << h << " " << k << " " << x << " " << y << '\n';
    for (int i = d - 1; i > 1; i--) {
        cc = cc * n % mod * n % mod;
        long long int res = h * c + k * cc;
        c = (cc * n - res) % mod;
        if (c < 0) c += mod;
    }
    cout << (x * c + y * cc) % mod;
}

• Subtask 1: 0 / 0
• Subtask 2: 0 / 7
• Subtask 3: 0 / 8
• Subtask 4: 0 / 15
• Subtask 5: 0 / 15
• Subtask 6: 0 / 20
• Subtask 7: 0 / 20
• Subtask 8: 0 / 15