Codeforces 911F Tree Destruction

Tree Destruction

先把直径扣出来， 然后每个点都和直径的其中一端组合， 这样可以保证是最优的。

#include<bits/stdc++.h>
#define LL long long
#define fi first
#define se second
#define mk make_pair
#define PLL pair<LL, LL>
#define PLI pair<LL, int>
#define PII pair<int, int>
#define SZ(x) ((int)x.size())
#define ull unsigned long long

using namespace std;

const int N = 2e5 + 7;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + 7;
const double eps = 1e-8;
const double PI = acos(-1);

int n, deg[N], deptha[N], depthb[N];
vector<int> G[N];
queue<int> que;
vector<pair<PII, int>> vc;

PII dfs(int u, int fa, int* depth) {
    if(fa) depth[u] = depth[fa] + 1;
    else depth[u] = 0;
    PII ans = mk(depth[u], u);
    for(auto& v : G[u]) {
        if(v == fa) continue;
        ans = max(ans, dfs(v, u, depth));
    }
    return ans;
}

int main() {
    scanf("%d", &n);
    for(int i = 2; i <= n; i++) {
        int u, v; scanf("%d%d", &u, &v);
        G[u].push_back(v);
        G[v].push_back(u);
        deg[u]++; deg[v]++;
    }
    int A = dfs(1, 0, deptha).se;
    int B = dfs(A, 0, deptha).se;
    dfs(B, 0, depthb);
    for(int i = 1; i <= n; i++)
        if(i != A && i != B && deg[i] == 1)
            que.push(i);
    LL ans = 0;
    while(!que.empty()) {
        int u = que.front(); que.pop();
        if(deptha[u] > depthb[u]) {
            ans += deptha[u];
            vc.push_back(mk(mk(A, u), u));
        } else {
            ans += depthb[u];
            vc.push_back(mk(mk(B, u), u));
        }
        for(auto& v : G[u])
            if(--deg[v] == 1)
                que.push(v);
    }
    que.push(B);
    while(!que.empty()) {
        int u = que.front(); que.pop();
        if(u == A) break;
        ans += deptha[u];
        vc.push_back(mk(mk(A, u), u));
        for(auto& v : G[u])
            if(--deg[v] == 1)
                que.push(v);
    }
    printf("%lld\n", ans);
    for(auto& t : vc) printf("%d %d %d\n", t.fi.fi, t.fi.se, t.se);
    return 0;
}

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