学步园

题意：(略) 注意求的长度是指树中最长的链，即树的直径   注意这里可能是不连通图

先 dfs 判断图中是否有环，然后求树的直径，做法是 以任意一点u，为根节点，dfs找到最远的点 start，再以该节点为根节点 dfs,找到的最远的点，它们的距离就是解。

code：

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <cassert>
#include <cstring>
#include <string>
#include <iomanip>
#include <queue>
#include <vector>
#include <set>
#include <stack>
#include <map>
#define LL long long
#define inf 0x3f3f3f3f
#define eps 1e-9
#define PA system("pause")
#define PI acos(-1.0)
using namespace std;
#define M 100005
int vis[M],head[M],dis[M];
struct edge
{
    int to, w, next;
}e[2000005];
int ecnt, n, st;
void addedge(int u, int v, int w)
{
    e[ecnt].to = v;
    e[ecnt].w = w;
    e[ecnt].next = head[u];
    head[u] = ecnt++;
}
void dfs1(int u) //从u 出发找最远的点
{
    vis[u] = 1;
    for(int i = head[u]; i != -1; i = e[i].next)
    {
        int v = e[i].to;
        if(!vis[v])
        {
            dis[v] = dis[u] + e[i].w;
            dfs1(v);
        }
    }
}
int dfs(int u,int fa) // dfs判断有没有环
{
    vis[u] = 1;
    for(int i = head[u]; i != -1; i = e[i].next)
    {
        int v = e[i].to;
        if(v != fa){
            if(vis[v])
                return 1;
            if(dfs(v, u))
                return 1;
        }
    }
    return 0;
}
bool check() 
{
    for(int i = 1; i <= n; i++)
    {
        if(!vis[i])
        {
            if(dfs(i, -1))
                return 1;
        }
    }
    return 0;
}
int getdia(int u) //找树的直径
{
    int i;
    for(i = 1; i <= n; i++)
        dis[i] = (i==u? 0:inf);
    dfs1(u);
    int len = -1;
    for(i = 1; i <= n; i++)
    {
        if(dis[i] != inf && dis[i] > len)
        {
            len = dis[i];
            st = i;
        }
    }
    for(i = 1; i <= n; i++)
        dis[i] = (i==st? 0:inf);
    memset(vis, 0, sizeof(vis));
    dfs1(st);
    len = -1;
    for(i = 1; i <= n; i++)
    {
        if(dis[i] != inf && dis[i] > len)
            len = dis[i];
    }
    return len;
}
int main()
{
    int m, x, y, z;
    while(scanf("%d%d",&n,&m) != EOF)
    {
        memset(head, -1, sizeof(head));
        ecnt = 0;
        while(m--)
        {
            scanf("%d%d%d",&x,&y,&z);
            addedge(x, y, z);
            addedge(y, x, z);
        }
        memset(vis,0,sizeof(vis));
        if(check())
            puts("YES");
        else
        {
            memset(vis, 0 ,sizeof(vis));
            int ans = -1;
            for(int i = 1; i <= n; i++)
            {
                if(!vis[i])
                {
                    ans = max(ans, getdia(i));
                }
            }
            printf("%d\n",ans);
        }
    }
    return 0;
}