题目链接
题解
题意很清楚，找一点使得最远点最近
如果是一棵树，就是直径中点
现在套上了一个环，我们把环单独拿出来
先求出环上每个点外向树直径更新答案，并同时求出环上每个点外向的最远距离(val[i])
首先要明白以下事实：
①删掉任意一条边不会使答案更优
②环上存在一条边，使得删掉后答案不变
所以我们要做的就是枚举这条边，然后快速求出断掉后的直径
如何快速求出一棵树的直径？
我们同样在剩余的环上找一个断边，直径只有两种情况：
①在断边的任意一侧
②经过断边
情况①，我们只需令(g[i])表示从断边一端(x)到(i)所形成的树的直径
则
[g[i] = max{g[i - 1],val[i] + dis(i,x)}
]
这个可以用单调队列优化
情况②，令(f[i])表示断边一端(u)到(i)中(val[i])的最大值
就可以直接求出了
然后问题就解决了
复杂度是(O(n))的
#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdio>
#include<cmath>
#include<map>
#define Redge(u) for (int k = h[u],to; k; k = ed[k].nxt)
#define REP(i,n) for (int i = 1; i <= (n); i++)
#define mp(a,b) make_pair<int,int>(a,b)
#define cls(s) memset(s,0,sizeof(s))
#define cp pair<int,int>
#define LL long long int
using namespace std;
const int maxn = 100005,maxm = 100005;
const LL INF = 10000000000000000ll;
inline int read(){
int out = 0,flag = 1; char c = getchar();
while (c < 48 || c > 57){if (c == '-') flag = -1; c = getchar();}
while (c >= 48 && c <= 57){out = (out << 3) + (out << 1) + c - 48; c = getchar();}
return out * flag;
}
int h[maxn],ne = 1;
struct EDGE{int to,nxt,f; LL w;}ed[maxn << 1];
inline void build(int u,int v,LL w){
ed[++ne] = (EDGE){v,h[u],1,w}; h[u] = ne;
ed[++ne] = (EDGE){u,h[v],1,w}; h[v] = ne;
}
int n,c[maxn],ci,incir[maxn],fa[maxn];
LL Lcir,sum[maxn],dep[maxn],Len[maxn],F[maxn];
void dfs(int u){
Redge(u) if ((to = ed[k].to) != fa[u]){
if (!dep[to]){
dep[to] = dep[u] + ed[k].w; fa[to] = u;
dfs(to);
}
else if (dep[to] < dep[u]){
ed[k].f = ed[k ^ 1].f = 0;
Lcir = dep[u] - dep[to] + ed[k].w;
for (int i = u; i != to; i = fa[i]){
c[++ci] = i;
sum[ci] = dep[i] - dep[fa[i]];
}
c[++ci] = to;
for (int i = ci; i; i--)
sum[i] = sum[i - 1];
for (int i = 1; i <= ci; i++)
sum[i] += sum[i - 1];
}
}
}
LL d[maxn];
int rt;
void DFS(int u,int flag){
if (d[u] > d[rt]) rt = u;
Redge(u) if (!incir[to = ed[k].to] && to != fa[u]){
fa[to] = u; d[to] = d[u] + ed[k].w;
DFS(to,flag);
if (flag) F[u] = max(F[u],F[to] + ed[k].w);
}
}
void dfs1(int u){
if (d[u] > d[rt]) rt = u;
Redge(u) if (ed[k].f && (to = ed[k].to) != fa[u]){
fa[to] = u; d[to] = d[u] + ed[k].w;
dfs1(to);
}
}
LL ans,f1[maxn],f2[maxn],g1[maxn],g2[maxn];
LL S[maxn],val[maxn],qv[maxn << 1];
int head,tail;
void work(){
for (int i = 1; i <= ci; i++){
S[i] = sum[i]; val[i] = F[c[i]];
}
for (int i = 1; i <= ci; i++){
f1[i] = max(f1[i - 1],val[i] + S[i]);
}
qv[head = tail = 0] = val[1] - S[1];
g1[1] = val[1];
for (int i = 2; i <= ci; i++){
g1[i] = max(g1[i - 1],val[i] + S[i] + qv[head]);
while (head <= tail && val[i] - S[i] >= qv[tail]) tail--;
qv[++tail] = val[i] - S[i];
}
for (int i = ci; i; i--){
f2[i] = max(f2[i + 1],val[i] + S[ci] - S[i]);
}
qv[head = tail = 0] = val[ci] + S[ci];
g2[ci] = val[ci];
for (int i = ci - 1; i; i--){
g2[i] = max(g2[i + 1],val[i] - S[i] + qv[head]);
while (head <= tail && val[i] + S[i] >= qv[tail]) tail--;
qv[++tail] = val[i] + S[i];
}
LL ret = INF,len = Lcir - S[ci];
for (int i = 1; i < ci; i++)
ret = min(ret,max(f1[i] + f2[i + 1] + len,max(g1[i],g2[i + 1])));
rt = 1; d[rt] = fa[rt] = 0;
dfs1(rt);
d[rt] = 0; fa[rt] = 0;
dfs1(rt);
ret = min(ret,d[rt]);
ans = max(ans,ret);
//REP(i,ci) printf("%d f1[] = %lld f2 = %lld g1 = %lld g2 = %lld
",i,f1[i],f2[i],g1[i],g2[i]);
}
int main(){
n = read(); int a,b,w;
REP(i,n) a = read(),b = read(),w = read(),build(a,b,w);
dep[1] = 1; dfs(1);
REP(i,ci) incir[c[i]] = true;
REP(i,ci){
int u = c[i]; incir[u] = false;
rt = u ; d[u] = fa[u] = 0;
DFS(u,1);
d[rt] = 0; fa[rt] = 0;
DFS(rt,0);
Len[u] = d[rt];
ans = max(ans,Len[u]);
incir[u] = true;
}
work();
printf("%.1lf
",ans / 2.0);
return 0;
}