[暑假集训]数据结构&简单图论专题1 C - Is It A Tree?

#include <iostream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <climits>
#include <algorithm>
#define MAXN 200007
#define REP(i,b,e) for(int i=b;i<e;i++)
#define DEBUG if(0)
using namespace std;
class POINT{public:
    vector<int> next;
    int mark;
    POINT(){RESET();}
    void RESET(){mark=0;next.clear();}
}p[MAXN];
int INDEX,from[MAXN],fnum=0,to[MAXN],tnum=0,flag,list[MAXN],lnum;
int dfs(int ind){
    if(!p[ind].mark)
        p[ind].mark=INDEX++;
    else {DEBUG{cout<<ind<<"-="<<p[ind].mark<<endl;};return 0;}//如果重复标号
    REP(i,0,p[ind].next.size()){
        DEBUG cout<<p[ind].next[i]<<" ";
        if(!dfs(p[ind].next[i]))return 0;
    }
    return 1;
}
int main(){
    int a,b,CASE=1;
    void init();
    while(scanf("%d %d",&a,&b),a+b>=0){
        init();
        do{
            //if(a==b && a!=0)continue;
            if(a==0 && b==0)break;
            DEBUG cout<<a<<"->"<<b<<endl;
            int flag2=0;
            REP(findi,0,p[a].next.size()){
                if(p[a].next[findi]==b){
                    flag2=1;break;
                }
            }if(flag2)continue;
            p[a].next.push_back(b);
            from[fnum++]=a;
            to[tnum++]=b;
            list[lnum++]=a;
            list[lnum++]=b;
            
        }while(scanf("%d %d",&a,&b));
        REP(fi,0,fnum){
            REP(ti,0,tnum){
                if(from[fi]==to[ti]){
                    from[fi]=0;
                }//筛选0入度点
            }
        }
        int rec=0;
        REP(fi,0,fnum){
            if(from[fi]>0){
                if(rec==0){
                    rec=from[fi];
                }else if(rec==from[fi]){
                    from[fi]=0;continue;
                }else{//出现两个0入度点
                    flag=0;
                    DEBUG cout<<from[fi]<<"+"<<endl;
                    break;
                }
                flag=dfs(from[fi]);
                if(!flag){
                    DEBUG cout<<from[fi]<<"="<<endl;
                    break;
                }
            }
        }
        REP(i,0,lnum){
            //确保全部标号
            int t=list[i];
            
            if(t>0 && !p[t].mark){
                flag=0;
                DEBUG cout<<t<<"-"<<endl;
            }
        }
        REP(i,0,lnum)if(list[i]>0)p[list[i]].RESET();
        printf("Case %d is%s a tree.\n",CASE++,flag?"":" not");
    }
    return 0;
}
void init(){
    INDEX=1;fnum=tnum=lnum=0;flag=1;
}

https://vjudge.net/contest/384914#problem/C