算法笔记--树的直径模板

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<vector>
#include<queue>
using namespace std;
#define ll long long
#define pb push_back
#define mem(a,b) memset(a,b,sizeof(a))

const int N=1e6+5;
int head[N];
int dis[N];
bool vis[N];
int cnt=0,b,mxn=0;
struct edge
{
    int to,w,next;
}edge[N];
void add_edge(int u,int v,int w)
{
    edge[cnt].to=v;
    edge[cnt].w=w;
    edge[cnt].next=head[u];
    head[u]=cnt++;
}
void bfs(int s)
{
    queue<int>q;
    q.push(s);
    int now,nxt;
    mxn=0;
    b=s;
    mem(dis,0);
    mem(vis,false);
    while(!q.empty())
    {
        now=q.front();
        q.pop();
        for(int i=head[now];~i;i=edge[i].next)
        {
            if(!vis[edge[i].to])
            {
                q.push(edge[i].to);
                vis[edge[i].to]=true;
                dis[edge[i].to]=dis[now]+edge[i].w;
                if(dis[edge[i].to]>mxn)
                {
                    mxn=dis[edge[i].to];
                    b=edge[i].to;
                }
            }
        }
    }
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    int u,v,w;
    mem(head,-1);
    //freopen("in.txt","r",stdin);
    while(cin>>u>>v>>w)add_edge(u,v,w),add_edge(v,u,w);
    bfs(1);
    //cout<<b<<‘ ‘<<mxn<<endl;
    bfs(b);
    cout<<mxn<<endl;
    //fclose(stdin);
    return 0;
}