Java 解法, 执行用时: 1874ms, 内存消耗: 70096K, 提交时间: 2022-06-19 19:26:58

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
public class Main {
    static Map<Integer, Integer> deadlock = new HashMap<>();
    static boolean isdeadlock = false;
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        Map<Integer, Integer> map = new HashMap<>();
        int q = in.nextInt();
        while(q --> 0){
           Integer occ = in.nextInt();
           Integer req = in.nextInt();
           map.put(occ, req);
        }
        map.forEach((k, v) ->{
            if (isDeadLock(k, map)) {
            	isdeadlock = true;
				return;
			}
        });
        System.out.println(isdeadlock ? "YES" : "NO");
        in.close();
    }
      
    public static boolean isDeadLock(Integer i, Map<Integer, Integer> map){
    	Integer val = map.get(i);
        if (val == null) {
        	deadlock.clear();
			return false;
		} else {
			if (deadlock.get(val) != null) {
				return true;
			}
			deadlock.put(i, val);
			map.put(i, null);
			return isDeadLock(val, map);
		}
    }
}

C++14(g++5.4) 解法, 执行用时: 125ms, 内存消耗: 7856K, 提交时间: 2019-10-26 15:50:06

#include<cstdio>
using namespace std;
int c[500010]={0};
int a[200010],cnt=0;
int n;
bool vis[500010]={0};
bool exist[500010]={0};
bool flag=0;
void dfs(int r){
	if(vis[r]==1)return;
	vis[r]=1;exist[r]=1;
	if(exist[c[r]]==1){
		flag=1;
		return;
	}
	if(c[r]!=0)
		dfs(c[r]);
	exist[r]=0;
}
int main(){
	scanf("%d",&n);
	int x,y;
	for(int i=1;i<=n;i++){
		scanf("%d%d",&x,&y);
		a[++cnt]=x;
		c[x]=y;
	}
	for(int i=1;i<=cnt;i++){
		if(flag==0&&vis[a[i]]==0){
			dfs(a[i]);
		}
	}
	if(flag==0){
		printf("NO");
	}
	else{
		printf("YES");
	}
	return 0;
}

C++11(clang++ 3.9) 解法, 执行用时: 542ms, 内存消耗: 15532K, 提交时间: 2019-10-26 17:39:26

#include <iostream>
#include <cstring>
#include <list>
#include <stdio.h>
using namespace std;

int main()
{
  list<int> l;
  int n,cnt=0;
  cin>>n;
  int k=n;
  while(n--)
  {
      int a,b;
      cin>>a>>b;
      l.insert(l.begin(),a);
      l.insert(l.begin(),b);
  }
   l.sort();
   l.unique();
  list<int> :: iterator it;

   for(it=l.begin();it!=l.end();it++)
   {
       cnt++;
       }

   if(cnt==k)
    cout<<"YES"<<endl;
   else
    cout<<"NO"<<endl;


    return 0;
}