求树的直径-模板

#include <bits/stdc++.h> 

using namespace std;

#define INF 0x3f3f3f3f
#define MAXN 1000010
#define MAXM 5010

inline int read()
{
    int x = 0,ff = 1;char ch = getchar();
    while(!isdigit(ch))
    {
        if(ch == '-') ff = -1;
        ch = getchar();
    }
    while(isdigit(ch))
    {
        x = (x << 1) + (x << 3) + (ch ^ 48);
        ch = getchar();
    }
    return x * ff;
}

inline void write(int x)
{
    if(x < 0) putchar('-'),x = -x;
    if(x > 9) write(x / 10);
    putchar(x % 10 + '0');
}

int a,b,dis[MAXN],vis[MAXN],ans,point;
int lin[MAXN],tot = 0;
struct edge
{
    int y,v,next;
}e[MAXN];

inline void add(int xx,int yy,int vv)
{
    e[++tot].y = yy;
    e[tot].v = vv;
    e[tot].next = lin[xx];
    lin[xx] = tot;
}

void BFS(int s)
{
    ans = 0;
    memset(dis,0,sizeof(dis));
    memset(vis,false,sizeof(vis));
    queue < int > q;
    q.push(s); vis[s] = true; point = s;
    while(!q.empty())
    {
        int x = q.front(); q.pop();
        for(int i = lin[x],y;i;i = e[i].next)
        {
            if(!vis[y = e[i].y])
            {
                q.push(y);
                vis[y] = true;
                dis[y] = dis[x] + e[i].v;
                if(dis[y] > ans)  ans = dis[y],point = y;
            }    
        }
    }
}

int main()
{
    a = read(); 
    for(int i = 1;i <= a;++i)
    {
        int x,y,v; 
        x = read(); y = read(); v = read();
        add(x,y,v);
        add(y,x,v);
    }
    BFS(1); BFS(point);
    write(ans);
    return 0;
}