Error-prone points:
- Seeking the endpoint diameter trees, at the deepest point should be acquired using the "> =" symbol.
- Need to be accompanied vis [i] array tree diameter when evaluated to ensure that each access point is only one (the source point of dis 0).
#include<cstdio>
#include<iostream>
#include<queue>
#include<cstring>
#define ll long long
using namespace std;
MAXN const int = 3E5, MaxM = MAXN * 2;
struct Edge{
int from,to,nxt;
ll w;
} E [MaxM];
int head[MAXN],edgeCnt=1;
void addEdge(int u,int v,ll w){
e[++edgeCnt].from=u;
e [edgeCnt] .to = v;
e[edgeCnt].w=w;
e[edgeCnt].nxt=head[u];
head[u]=edgeCnt;
}
ll dep[MAXN];
bool vis[MAXN];
int n;
ll length; // tree diameter
int bfs (int s) {// return the deepest point
memset(dep,0,sizeof(dep));
memset(vis,0,sizeof(vis));
queue<int> q;
q.push(s);
want [s] = 1;
while(!q.empty()){
int nowU=q.front();
q.pop();
for(int i=head[nowU];i;i=e[i].nxt){
int nowV=e[i].to;
if(!vis[nowV]){
dep [nowV] = dep [Hade] + e [i] .w;
view [nowV] = 1;
q.push(nowV);
}
}
}
int years = 0;
for(int i=1;i<=n;i++)
if(dep[ans]<=dep[i]){//注意( "=" )
years = i;
length=dep[i];
}
return years;
}
ll dis1[MAXN];
void dfs1(int x,int in_edge){
for(int i=head[x];i;i=e[i].nxt){
if(i==(in_edge^1))continue;
int nowV=e[i].to;
dis1[nowV]=dis1[x]+e[i].w;
dfs1(nowV,i);
}
}
ll dis2[MAXN];
void dfs2(int x,int in_edge){
for(int i=head[x];i;i=e[i].nxt){
if(i==(in_edge^1))continue;
int nowV=e[i].to;
dis2[nowV]=dis2[x]+e[i].w;
dfs2(nowV,i);
}
}
int main () {
int m;
scanf("%d%d",&n,&m);
for(int i=1;i<=m;i++){
you and, v;
ll w;
cin>>u>>v>>w;
addEdge(u,v,w);
addEdge(v,u,w);
}
int st=bfs(1);
int ed = bfs (st); // end points of the diameter of the tree
dfs1(st,0);
dfs2(ed,0);
ll ans=0;
for(int i=1;i<=n;i++)
// worst case; ans = max (ans, min (dis1 [i], dis2 [i]))
years = years + length;
cout<<ans<<endl;
return 0;
}