| # | Time | Username | Problem | Language | Result | Execution time | Memory |
|---|---|---|---|---|---|---|---|
| 1302296 | amrharb | Race (IOI11_race) | C++20 | 0 ms | 0 KiB |
#include<bits/stdc++.h>
#define int long long
using namespace std;
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int n, k;
cin >> n >> k;
vector<vector<int> > adj(n);
for (int i = 1; i < n; i++) {
int x, y;
cin >> x >> y, --x, --y;
adj[x].push_back(y);
adj[y].push_back(x);
}
vector<bool> visited(n);
vector<int> sz(n);
function<void(int,int)> dfs_sz = [&](int u,int p) {
sz[u] = 1;
for (auto v: adj[u]) {
if (v == p || visited[v])continue;
dfs_sz(v, u);
sz[u] += sz[v];
}
};
vector<int> dist;
function<void(int,int,int)> dfs = [&](int u,int p,int cur) {
dist.push_back(cur);
for (auto v: adj[u]) {
if (v == p || visited[v])continue;
dfs(v, u, cur + 1);
}
};
function<int(int,int,int)> find_centroid = [&](int u,int p,int size) {
for (auto v: adj[u]) {
if (v == p || visited[v])continue;
if (sz[v] > size / 2)
return find_centroid(v, u, size);
}
return u;
};
vector<int> cnt(n + 1);
int ans = 0;
function<void(int)> decompose = [&](int u) {
dfs_sz(u, -1);
int centroid = find_centroid(u, -1, sz[u]);
visited[centroid] = true;
cnt[0] = 1;
for (auto v: adj[centroid]) {
if (visited[v])continue;
dist.clear();
dfs(v, centroid, 1);
for (auto d: dist) {
if (d <= k)
ans += cnt[k - d];
}
for (auto d: dist) {
cnt[d]++;
}
}
for (int i = 0; i <= sz[u]; i++)cnt[i] = 0;
for (auto v: adj[centroid]) {
if (visited[v])continue;
decompose(v);
}
};
decompose(0);
cout << ans;
return 0;
}