01:59(160)

hub/hub.cpp

@@ -0,0 +1,104 @@
+#include<bits/stdc++.h>
+using namespace std;
+int c, t;
+int n, k;
+vector<int> fri[1000005];
+int fa[1000005];
+int son[1000005];
+int cnt[1000005];
+int top[1000005];
+int dep[1000005];
+int depmaxson[1000005];
+void dfs1(int now, int from) {
+    cnt[now] = 1;
+    depmaxson[now] = dep[now];
+    for(int i : fri[now]) {
+        if(i == from) continue;
+        fa[i] = now;
+        dep[i] = dep[now] + 1;
+        dfs1(i, now);
+        depmaxson[now] = max(depmaxson[now], depmaxson[i]);
+        cnt[now] += cnt[i];
+        if(cnt[son[now]] < cnt[i]) son[now] = i;
+    }
+}
+void dfs2(int now, int from) {
+    top[now] = from;
+    if(son[now]) dfs2(son[now], from);
+    for(int i : fri[now]) {
+        if(i == fa[now] || i == son[now]) continue;
+        dfs2(i, i);
+    }
+}
+int lca(int a, int b) {
+    while(top[a] != top[b]) {
+        if(dep[top[a]] < dep[top[b]]) swap(a, b);
+        a = fa[top[a]];
+    }
+    if(dep[a] < dep[b]) swap(a, b);
+    return b;
+}
+bool chosen[1000005];
+struct BCJ{
+    int fa[1000005];
+    int find(int x) {
+        if(fa[x] == x) return x;
+        return fa[x] = find(fa[x]);
+    }
+}bcj;
+void dfs0(int now) {
+    for(int i : fri[now]) {
+        if(i == fa[now]) continue;
+        if(chosen[i] && chosen[now]) bcj.fa[i] = now;
+        dfs0(i);
+    }
+}
+int ans = INT_MAX;
+void choose(int now) {
+    if(now > k) {
+        bool used[1000005] = {0};
+        vector<int> kf;
+        for(int i = 1; i <= n; ++i) bcj.fa[i] = i;
+        dfs0(1);
+        int cnt = 0;
+        for(int i = 1; i <= n; ++i) {
+            if(!used[bcj.find(i)]) used[bcj.find(i)] = 1, cnt++, kf.push_back(bcj.find(i));
+            if(cnt > 2) return;
+        }
+        int l = lca(kf[0], kf[1]);
+        ans = min(ans, depmaxson[kf[0]] + depmaxson[kf[1]] - 2 * dep[l]);
+    }
+    for(int i = 1; i <= n; ++i) {
+        if(!chosen[i]) chosen[i] = 1, choose(now + 1), chosen[i] = 0;
+    }
+}
+int main() {
+    freopen("hub.in", "r", stdin);
+    freopen("hub.out", "w", stdout);
+    scanf("%d%d", &c, &t);
+    while(t--) {
+        scanf("%d%d", &n, &k);
+        for(int i = 1, u, v; i < n; ++i) {
+            scanf("%d%d", &u, &v);
+            fri[u].push_back(v);
+            fri[v].push_back(u);
+        }
+        if(k == n) {
+            printf("0\n");
+            for(int i = 1; i <= n; ++i) fri[i].clear();
+        }
+        dfs1(1, 1);
+        dfs2(1, 1);
+        choose(1);
+        printf("%d\n", ans);
+        for(int i = 1; i <= n; ++i) fri[i].clear();
+        ans = INT_MAX;
+    }
+    return 0;
+}
+/*
+g++ hub/hub.cpp -o hub/hub -std=c++14 -O2 -static -Wall && cp hub/hub ~ && chmod +x ~/hub && ~/hub
+
+对于两个连通块中的点,距离为道路条数(lca)
+
+*/