樹的直徑模闆
http://codevs.cn/problem/1814/最長鍊.模闆題
對于樹上的最長鍊,有且僅有兩種情況:
![](https://img.laitimes.com/img/9ZDMuAjOiMmIsIjOiQnIsICdzFWRoRXdvN1LclHdpZXYyd2LcBzNvwVZ2x2bzNXak9CX90TQNNkRrFlQKBTSvwFbslmZvwFMwQzLcVmepNHdu9mZvwFVywUNMZTY18CX052bm9CX90keNFTTU9kMZpXTmZEWjZXUYpVd1kmYr50MZV3YyI2cKJDT29GRjBjUIF2LcRHelR3LcJzLctmch1mclRXY39zN1MTMwkDNyITMxATM3EDMy8CX0Vmbu4GZzNmLn9Gbi1yZtl2Lc9CX6MHc0RHaiojIsJye.jpg)
#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
using namespace std;
int n,tot,a,b,ans,pos;
int first[],nxt[];
struct edge
{
int u,v;
}l[];
void build(int f,int t)
{
l[++tot]=(edge){f,t};
nxt[tot]=first[f];
first[f]=tot;
}
void dfs(int k,int fa,int d)
{
if(d>ans)
{
ans=d;
pos=k;
}
for(int i=first[k];i!=-;i=nxt[i])
{
int x=l[i].v;
if(x==fa)
continue;
dfs(x,k,d+);
}
}
int main()
{
memset(first,-,sizeof(first));
scanf("%d",&n);
for(int i=;i<=n;i++)
{
scanf("%d%d",&a,&b);
if(a!=)
{
build(i,a);
build(a,i);
}
if(b!=)
{
build(i,b);
build(b,i);
}
}
dfs(,,);//第一遍DFS找出樹上深度最深的點
ans=;
dfs(pos,,);//第二遍DFS由深度最深的點向上找最長鍊
printf("%d",ans);
return ;
}