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;

}