POJ 1985 Cow Marathon (树的直径) 模板

树的直径题目。两次BFS

//9186307 NKHelloWorld 1985 Accepted 1824K 141MS C++ 1490B 2011-08-18 10:56:49
//9186317 NKHelloWorld 1985 Accepted 1824K 110MS C++ 1565B 2011-08-18 10:57:21
//9186348 NKHelloWorld 1985 Accepted 1888K 141MS G++ 1644B 2011-08-18 10:59:14
#include <cstdio>
#include <cstring>
#define MAXN 81000
int n,m;
struct EDGE
{
    int to,dis,next;
}edge[MAXN];
int pt_edge = 0,index[41000],endpt = 1,ans = 0;

int que[MAXN],dis[MAXN];
bool visited[MAXN];
int bfs(int start)
{
    int j;
    int tail = 0,head = 0;
    memset(visited,false,sizeof(visited));
    dis[tail] = 0;
    que[tail++] = start;
    visited[start] = true;
    while(head < tail)
    {
        for(j=index[que[head]];j!=-1;j=edge[j].next)
        {
            if(visited[edge[j].to]==false)
            {
                dis[tail] = dis[head] + edge[j].dis;
                que[tail] = edge[j].to;
                visited[que[tail]] = true;
                if(dis[tail] > ans)
                {
                    ans = dis[tail];
                    endpt = que[tail];
                }
                tail++;
            }
        }
        head++;
    }
    return ans;
}

int main()
{
    int i,j,f1,f2,dis;
    char dir[5];
    memset(index,-1,sizeof(index));
    scanf("%d%d",&n,&m);
    for(i=1;i<=m;i++)
    {
        scanf("%d%d%d%s",&f1,&f2,&dis,dir);
        edge[pt_edge].next = index[f1]; index[f1] = pt_edge;
        edge[pt_edge].to = f2;  edge[pt_edge++].dis = dis;
        edge[pt_edge].next = index[f2]; index[f2] = pt_edge;
        edge[pt_edge].to = f1;  edge[pt_edge++].dis = dis;
    }
    bfs(1);
    printf("%d\n",bfs(endpt));
    return 0;
}