树的直径 Codeforces Beta Round #14 (Div. 2) D. Two Paths

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <algorithm>
#include <set>
using namespace std;
typedef long long ll;
typedef unsigned long long Ull;
#define MM(a,b) memset(a,b,sizeof(a));
const double eps = 1e-10;
const int inf = 0x3f3f3f3f;
const double pi=acos(-1);
const int mod=100000000;
ll max(ll a,ll b)
{return a>b?a:b;};
int min(int a,int b)
{return a<b?a:b;};

struct Edge{
  int u,v;
}e[205];

vector<int> G[205];
int vis[205],step[205],s,len;

int bfs(int a,int pre)
{
    int s=a;
    len=0;
    MM(vis,0);
    MM(step,0);
    queue<int> q;
    q.push(a);
    vis[pre]=vis[a]=1;
    while(q.size())
    {
        int u=q.front();q.pop();
        for(int i=0;i<G[u].size();i++)
        {
            int v=G[u][i];
            if(vis[v]) continue;
            q.push(v);
            vis[v]=1;
            step[v]=max(step[v],step[u]+1);
            if(step[v]>len)
            {
                len=step[v];
                s=v;
            }
        }
    }
    return s;
}//求树的直径len模板

int main()
{
    int n,u,v;
    while(~scanf("%d",&n))
    {
       for(int i=1;i<=n;i++) G[i].clear();
       for(int i=1;i<n;i++)
       {
           scanf("%d %d",&e[i].u,&e[i].v);
           G[e[i].u].push_back(e[i].v);
           G[e[i].v].push_back(e[i].u);
       }

       int ans=0;
       for(int i=1;i<n;i++)
       {
           int u=e[i].u,v=e[i].v;
           bfs(bfs(u,v),v);
           int x=len;
           bfs(bfs(v,u),u);
           x*=len;
           ans=max(ans,x);
       }
       printf("%d\n",ans);
    }
    return 0;
}