树的直径
并查集判环，SPFA判环一直T。。。。（写的太挫了估计。。。）
#include <cstdio>
#include <cstring>
#include <iostream>
#include <vector>
#include <queue>
using namespace std;
int const MAXN = 100010;
typedef struct EDGE{
int to;
int w;
}Edge;
vector <Edge> edge[MAXN];
queue <int> q;
int cnt[MAXN],vis[MAXN],dis[MAXN];
int father[MAXN];
int n,m;
void Init(){
for(int i = 0;i < MAXN;i++){
father[i] = i;
edge[i].clear();
}
}
void Add(int u,int v,int w){
Edge temp;
temp.w = w;
temp.to = v;
edge[u].push_back(temp);
temp.to = u;
edge[v].push_back(temp);
}
int Find(int x){
int temp,y;
y = x;
while(y != father[y]){
y = father[y];
}
while(x != father[x]){
temp = father[x];
father[x] = y;
x = temp;
}
return y;
}
void Union(int u,int v){
int uu = Find(u);
int vv = Find(v);
if(uu != vv) father[uu] = vv;
}
void Bfs(int start){
while(!q.empty()){q.pop();};
memset(vis,0,sizeof(vis));
memset(dis,-1,sizeof(dis));
vis[start] = 1;
dis[start] = 0;
q.push(start);
while(!q.empty()){
int top = q.front();
q.pop();
for(int i = 0;i < edge[top].size();i++){
int to = edge[top][i].to;
if(!vis[to]){
vis[to] = 1;
dis[to] = dis[top] + edge[top][i].w;
q.push(to);
}
}
}
}
int Count(int start){
int u = start, maxx = 0;
Bfs(u);
for(int i = 1;i <= n;i++){
if(dis[i] > maxx){
u = i;
maxx = dis[i];
}
}
Bfs(u);
maxx = 0;
for(int i = 1;i <= n;i++){
if(dis[i] > maxx) maxx = dis[i];
}
return maxx;
}
int main(){
while(~scanf("%d%d",&n,&m)){
Init();
int u,v,w,flag = 0;
for(int i = 0;i < m;i++){
scanf("%d%d%d",&u,&v,&w);
Add(u,v,w);
if(Find(u) == Find(v)) flag = 1;
Union(u,v);
}
if(flag) printf("YES\n");
else{
printf("%d\n",Count(u));
}
}
return 0;
}