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