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OI-codes/LibreOJ/2482/2482.cpp

92 lines
1.5 KiB
C++

#include <iostream>
#include <vector>
using std::cin;
using std::cout;
const char endl = '\n';
const int N = 1e6 + 5;
int n, t, m, fa[N], sum[N], f[N], ans;
bool vis[N];
std::vector<int> g[N];
void dfs(int u, int _f) {
int max1 = 0, max2 = 0;
fa[u] = _f;
if (u != t) {
sum[u] = sum[_f] + g[u].size() - 1 - (u != m);
}
for (const int &v : g[u]) {
if (v == _f) continue;
dfs(v, u);
if (f[v] > max1) {
max2 = max1;
max1 = f[v];
} else if (f[v] > max2) {
max2 = f[v];
}
}
f[u] = max2 + g[u].size() - 1;
}
bool check(int x) {
for (int i = m, cnt = 1; i != t; i = fa[i], cnt++) {
int t = 0;
for (const int &v : g[i]) {
if (vis[v] || sum[i] + f[v] <= x) continue;
if (!cnt) return false;
t++, cnt--;
}
x -= t;
}
return x >= 0;
}
int main() {
std::ios::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> t >> m;
for (int i = 1, u, v; i < n; i++) {
cin >> u >> v;
g[u].push_back(v);
g[v].push_back(u);
}
dfs(t, 0);
for (int i = m; i; i = fa[i]) {
vis[i] = true;
}
int l = 0, r = n << 1;
while (l <= r) {
int mid = l + r >> 1;
if (check(mid)) {
r = mid - 1;
ans = mid;
} else {
l = mid + 1;
}
}
cout << ans << endl;
return 0;
}