【树的直径】 ZJU 3820 Building Fire Stations

先找到树的直径 len  再将直径均匀分成两根。。

如果不能平分则需要比较 max（（len/2 和len/2-1 ），（len/2 和len/2+1 ））

#include <cstdio>#include <cstring>#include <cstdlib>#include <string>#include <iostream>#include <algorithm>#include <sstream>#include <cmath>using namespace std;#include <queue>#include <stack>#include <vector>#include <deque>#define cler(arr, val)    memset(arr, val, sizeof(arr))#define FOR(i,a,b)  for(int i=a;i<=b;i++)#define IN   freopen ("in.txt" , "r" , stdin);#define OUT  freopen ("out.txt" , "w" , stdout);typedef long long  LL;const int MAXN = 200000;const int MAXM = 200000;const int INF = 0x3f3f3f3f;const int mod = 1000000007;const double eps= 1e-8;#define lson l,m,rt<<1#define rson m+1,r,rt<<1|1struct Edge{    int to,next;} edge[MAXN*2];int head[MAXN],pre[MAXN],d[MAXN],tol;bool vis[MAXN];void init(){    tol=0;    memset(head,-1,sizeof(head));}void addedge(int u,int v){    edge[tol].to=v,edge[tol].next=head[u];    head[u]=tol++;    edge[tol].to=u,edge[tol].next=head[v];    head[v]=tol++;}int bfs(int u,int no){    int point=u,big=0;    memset(vis,false,sizeof(vis));    queue<int>q;    q.push(u);    vis[u]=vis[no]=true;    pre[u]=-1;    d[u]=1;    while(!q.empty())    {        u=q.front();        q.pop();        for(int i=head[u]; ~i; i=edge[i].next)        {            int v=edge[i].to;            if(!vis[v])            {                d[v]=d[u]+1;                pre[v]=u;                if(d[v]>big)                {                    big=d[v];                    point=v;                }                vis[v]=true;                q.push(v);            }        }    }    return point;}void solve(int s,int e,int &out1,int &out2,int &ans){    int u,v;    u=bfs(s,e);    v=bfs(u,e);    int star[MAXN];    int len1=0,tmp;    tmp=v;    while(tmp!=-1)    {        star[len1++]=tmp;        tmp=pre[tmp];    }    out1=star[(len1)/2];    ans=(len1)/2;    u=bfs(e,s);    v=bfs(u,s);    int len2=0;    tmp=v;    while(tmp!=-1)    {        star[len2++]=tmp;        tmp=pre[tmp];    }    out2=star[(len2)/2];    ans=max(ans,len2/2);}int main(){#ifndef ONLINE_JUDGE    freopen("in.txt", "r", stdin);    //  freopen("out.txt", "w", stdout);#endif    int t,a,b,n;    scanf("%d", &t);    while(t--)    {        int star[MAXN];        init();        scanf("%d", &n);        for(int i=0; i<n-1; i++)        {            scanf("%d%d",&a,&b);            addedge(a,b);        }        int u=bfs(1,n+1);        int v=bfs(u,n+1);        int tmp,num=0,ans,out1,out2;        int len=0;        tmp=v;        while(tmp!=-1)        {            star[len++]=tmp;            tmp=pre[tmp];        }        int s=star[(len)/2];        int e=star[(len)/2-1];        solve(s,e,out1,out2,ans);        if(len%2==1)        {            int s=star[(len)/2];            int e=star[(len)/2+1];            int out3,out4,ans2;            solve(s,e,out3,out4,ans2);            if(ans2>ans)                out1=out3,out2=out4;        }        printf("%d %d %d\n",ans,out1,out2);    }    return  0;}