POJ_1985_Cow Marathon(树的直径)

#include<iostream>
#include<cstring>
#include<cmath>
#include<queue>
#include<cstdio>
#define mt(a,b) memset(a,b,sizeof(a));
using namespace std;

const int M = 45678;
int n,m;

struct G {
struct E {
int u,v,w,next;
} e[M*4];
int le,head[M];
void init() {
le=0;
mt(head,-1);
}
void add(int u,int v,int w) {
e[le].u=u;
e[le].v=v;
e[le].w=w;
e[le].next=head[u];
head[u]=le++;
}
} g;

int longest;
int dis[M];
bool used[M];
int bfs(int start) {
mt(dis,-1);
dis[start] = 0;
mt(used,false);
queue<int> q;
q.push(start);
used[start] = true;
while(!q.empty()) {
int u = q.front();
q.pop();
for(int i=g.head[u]; ~i; i=g.e[i].next) {
int v = g.e[i].v;
int w = g.e[i].w;
if(dis[v] < dis[u] + w && !used[v]) {
dis[v] = dis[u] + w;
q.push(v);
used[v] = true;
}
}
}
longest = -1;
int point = -1;
for(int i=1; i<=n; i++) {
if(longest < dis[i]) {
longest = dis[i];
point = i;
}
longest = max(longest,dis[i]);
}
return point;
}

int main() {
while(~scanf("%d%d",&n,&m)) {
g.init();
int u,v,w;
char d[5];
for(int i=0; i<m; i++) {
scanf("%d%d%d%s",&u,&v,&w,d);
g.add(u,v,w);
g.add(v,u,w);
}

int id = bfs(1);

//        for(int i=1;i<=n;i++){
//            printf("%d ",dis[i]);
//        }
//        puts("");

id = bfs(id);
printf("%d\n",dis[id]);

}
return 0;
}