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OI-codes/Luogu/P2634/P2634.cpp

99 lines
1.7 KiB
C++

#include <iostream>
#include <algorithm>
#include <limits>
#include <vector>
using std::cin;
using std::cout;
const char endl = '\n';
const int N = 2e4 + 5;
int n, ans;
int root, siz[N], max[N];
int p[10];
bool vis[N];
std::vector<std::pair<int, int>> g[N];
void find(int u, int fa, int tot) {
siz[u] = 1;
max[u] = 0;
for (auto e : g[u]) {
int v = e.first,
w = e.second;
if (v == fa || vis[v]) continue;
find(v, u, tot);
siz[u] += siz[v];
max[u] = std::max(max[u], siz[v]);
}
max[u] = std::max(max[u], tot - siz[u]);
if (max[u] < max[root]) root = u;
}
void dis(int u, int fa, int sum) {
p[sum % 3]++;
for (auto e : g[u]) {
int v = e.first,
w = e.second;
if (v == fa || vis[v]) continue;
dis(v, u, sum + w);
}
}
int calc(int u, int w) {
std::fill(p, p + 3, 0);
dis(u, 0, w);
return p[0] * p[0] + 2 * p[1] * p[2];
}
void solve(int u) {
vis[u] = true;
ans += calc(u, 0);
for (auto e : g[u]) {
int v = e.first,
w = e.second;
if (vis[v]) continue;
ans -= calc(v, w);
max[root = 0] = std::numeric_limits<int>::max();
find(v, 0, siz[v]);
solve(root);
}
}
int main() {
std::ios::sync_with_stdio(false);
cin >> n;
for (int i = 1, u, v, w; i < n; i++) {
cin >> u >> v >> w;
g[u].push_back(std::make_pair(v, w % 3));
g[v].push_back(std::make_pair(u, w % 3));
}
max[root = 0] = n;
find(1, 0, n);
solve(root);
int gcd = std::__gcd(ans, n * n);
cout << ans / gcd << '/' << n * n / gcd << endl;
return 0;
}