1056. Computer Net

http://acm.timus.ru/problem.aspx?space=1&num=1056

求树的圆心

两遍 bfs 找到直径 然后圆心有一个或两个

代码:

#include<iostream>
#include<stdio.h>
#include<string.h>
#include<math.h>
#include<algorithm>
#include<vector>
#include<set>
#include<map>
#include<string>
#include<queue>
#include<stack>
#include <iomanip>
using namespace std;
#define LL long long
const int INF=0x3f3f3f3f;
const int N=10006;
int head[N],I;
struct node
{
    int j,next;
}side[N*2];
int n;
int f[N];
vector<int>road;
void add(int i,int j)
{
    side[I].j=j;
    side[I].next=head[i];
    head[i]=I++;
}
int bfs(int s)
{
    int dist[N];
    memset(dist,-1,sizeof(dist));
    queue<int>qt;
    qt.push(s);
    dist[s]=0;
    int k=s;
    while(!qt.empty())
    {
        int x=qt.front();qt.pop();
        for(int t=head[x];t!=-1;t=side[t].next)
        {
            int j=side[t].j;
            if(dist[j]==-1)
            {
                dist[j]=dist[x]+1;
                f[j]=x;
                if(dist[j]>dist[k])
                k=j;
                qt.push(j);
            }
        }
    }
    road.clear();
    int tmp=k;
    while(tmp!=s)
    {
        road.push_back(tmp);
        tmp=f[tmp];
    }
    road.push_back(s);
    return k;
}
int main()
{
    //freopen("data.in","r",stdin);
    while(cin>>n)
    {
        memset(head,-1,sizeof(head));
        I=0;
        for(int i=2;i<=n;++i)
        {
            int pre;
            cin>>pre;
            add(pre,i);
            add(i,pre);
        }
        bfs(bfs(1));
        int m=road.size();

        if(m&1)
        cout<<road[m/2]<<endl;
        else
        cout<<min(road[m/2],road[m/2]-1)<<" "<<max(road[m/2],road[m/2]-1)<<endl;
    }
    return 0;
}