hihocoder 1050

求树的直径(即最远的两个节点的距离），原理就是我们从任意一个点u出发，bfs至一个最远的点v，那么可以证明v是树直径上的一点，那么我们再次从v开始bfs一次，能搜到的最远的那个点就是树直径的另外一端，则它们之间的距离就是树的直径。

#include<stdio.h>
#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<iomanip>
#include<vector>
#include<time.h>
#include<queue>
#include<stack>
#include<iterator>
#include<math.h>
#include<stdlib.h>
#include<limits.h>
#include<map>
//#define ONLINE_JUDGE
#define eps 1e-8
#define INF 0x7fffffff
#define FOR(i,a) for((i)=0;i<(a);(i)++)
#define MEM(a) (memset((a),0,sizeof(a)))
#define sfs(a) scanf("%s",a)
#define sf(a) scanf("%d",&a)
#define sfI(a) scanf("%I64d",&a)
#define pf(a) printf("%d\n",a)
#define pfI(a) printf("%I64d\n",a)
#define pfs(a) printf("%s\n",a)
#define sfd(a,b) scanf("%d%d",&a,&b)
#define sft(a,b,c)scanf("%d%d%d",&a,&b,&c)
#define for1(i,a,b) for(int i=(a);i<b;i++)
#define for2(i,a,b) for(int i=(a);i<=b;i++)
#define for3(i,a,b)for(int i=(b);i>=a;i--)
#define MEM1(a) memset(a,0,sizeof(a))
#define MEM2(a) memset(a,-1,sizeof(a))
const double PI=acos(-1.0);
template<class T> T gcd(T a,T b){return b?gcd(b,a%b):a;}
template<class T> T lcm(T a,T b){return a/gcd(a,b)*b;}
template<class T> inline T Min(T a,T b){return a<b?a:b;}
template<class T> inline T Max(T a,T b){return a>b?a:b;}
using namespace std;
#define ll __int64
int n,m;
#define N 100010
struct Edge{
int from,to;
Edge(int from,int to){
this->from = from;
this->to = to;
}
};
struct Node{
int node;
int len;
};
vector<Edge> edges;
vector<int>v
;
void Init(){
edges.clear();
for(int i=0;i<=n;i++)
v[i].clear();
}
void addEdge(int f,int t){
edges.push_back(Edge(f,t));
edges.push_back(Edge(t,f));
m = edges.size();
v[f].push_back(m-2);
v[t].push_back(m-1);
}
int vis
;
queue<Node>q;
int st;
int maxlen;
void bfs(int s){
memset(vis,0,sizeof vis);
while(!q.empty())
q.pop();
Node tmp;
Node tmp1;
tmp.node = s;
tmp.len = 0;
q.push(tmp);
vis[s] = 1;
maxlen = 0;
while(!q.empty()){
tmp = q.front();
int u = tmp.node;
q.pop();
if(tmp.len>maxlen){
maxlen = tmp.len;
st = tmp.node;
}
int size = v[u].size();
for(int i=0;i<size;i++){
Edge edge = edges[v[u][i]];
if(vis[edge.to])
continue;
vis[edge.to] = 1;
tmp1.node = edge.to;
tmp1.len = tmp.len + 1;
q.push(tmp1);
}
}
}
int main(){
#ifndef ONLINE_JUDGE
freopen("in.txt","r",stdin);
// freopen("out.txt","w",stdout);
#endif
while(sf(n)!=EOF){
Init();
int u,v;
for(int i=1;i<n;i++){
sfd(u,v);
addEdge(u,v);
}
bfs(1); //从任意一个节点开始bfs
bfs(st); //st是从1 bfs到的最远的节点，即直径上的某一点
printf("%d\n",maxlen);
}
return 0;
}