URAL 1056 Computer Net（最短路）

#include <iostream>
#include <cstring>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <string>
#include <stack>
#include <queue>
#include <vector>
#define inf 0x3f3f3f3f
#define met(a,b) memset(a,b,sizeof a)
typedef long long ll;
using namespace std;
const int N = 10005;
const int M = 24005;
int vis[N],dis[N],pre[N],head[N];
int n,m,tot=0,son,maxn;
struct EDG{int to,next;}edg[N*N];
void add(int u,int v){
    edg[tot].to=v;edg[tot].next=head[u];head[u]=tot++;
}
void bfs(int s) {
    met(vis,0);met(dis,inf);met(pre,0);
    dis[s] = 0;
    vis[s] = 1;maxn=0;
    queue<int>q;q.push(s);
    while(!q.empty()){
        int u=q.front();q.pop();vis[u]=0;
        for(int i=head[u];i!=-1;i=edg[i].next){
            int v=edg[i].to;
            if(dis[v]>dis[u]+1){
                dis[v]=dis[u]+1;maxn=max(maxn,dis[v]);pre[v]=u;
                if(!vis[v]){
                    q.push(v);vis[v]=1;
                }
            }
        }
        son=u;
    }
}
int main() {
    met(head,-1);
    int a[N],cnt=0;
    scanf("%d",&n);
    for(int i=2;i<=n;i++){
        scanf("%d",&m);add(i,m);add(m,i);
    }
    bfs(1);int s=son;
    bfs(son);int t=son;//printf("!!%d %d\n",s,t);
    if(maxn&1){
        int x=maxn/2;
        while(t){
            if(dis[t]==x||dis[t]==x+1)a[cnt++]=t;
            t=pre[t];
        }
    }
    else {
        int x=maxn/2;
        while(t){
            if(dis[t]==x)a[cnt++]=t;
            t=pre[t];
        }
    }
    sort(a,a+cnt);
    for(int i=0;i<cnt;i++)printf("%d ",a[i]);
    printf("\n");
    return 0;
}