Poj1985 cow marathon --- tree diameter

The diameter of the tree is the length of the longest path in the tree.

Two DFS requests are used to obtain the farthest point P from any point for the first time,

The second time we get the farthest point from P, this path is the longest path, that is, the longest path.

#include <iostream>#include <cstring>#include <string>#include <cstdio>#include <cmath>#include <algorithm>#include <vector>#include <queue>#include <map>#define inf 0x3f3f3f3f#define eps 1e-6#define ll __int64using namespace std;struct node{    int v,w,next;}e[80010];int n,m,ans,p,h,head[40010],vis[40010];void addedge(int a,int b ,int c){    e[h].v=b;    e[h].w=c;    e[h].next=head[a];    head[a]=h++;}void init(){    memset(head,-1,sizeof head);    h=0;}void dfs(int x,int num){    vis[x]=1;    if(num>ans) ans=num,p=x;    for(int i=head[x];i!=-1;i=e[i].next)    {        if(!vis[e[i].v])            dfs(e[i].v,num+e[i].w);    }}int main(){    int a,b,c;    char s[10];    while(~scanf("%d%d",&n,&m))    {        init();        while(m--)        {            scanf("%d%d%d%s",&a,&b,&c,s);            addedge(a,b,c);            addedge(b,a,c);        }        ans=0;        memset(vis,0,sizeof vis);        dfs(1,0);        memset(vis,0,sizeof vis);        dfs(p,0);        printf("%d\n",ans);    }    return 0;}