TopologicalSort.txt

//#include <bits/stdc++.h>
#include <string>
#include <vector>
#include <math.h>
#include <map>
#include <string>
#include <cstdio>
#include <cmath>
#include <iostream>
#include <cmath>
#include <set>
#include <queue>


#define vpii vector< pair<int,int> >
#define vi vector<int>

#define for0(i,a) for(int i=0;i<a;i++)
#define pii              pair <int,int>
#define pll              pair <long long,long long>
#define sc               scanf
#define pf               printf
#define Pi               2*acos(0.0)
#define ms(a,b)          memset(a, b, sizeof(a))
#define pb(a)            push_back(a)
#define MP               make_pair
#define db               double
#define ll               long long
#define ull              unsigned long long
#define f                first
#define s                second
#define sqr(x)           (x)*(x)
#define VI               vector <int>
#define DBG              pf("Hi\n")
#define MOD              1000000007
#define SZ(a)            (int)a.size()
#define sf(a)            scanf("%d",&a)
#define sfl(a)           scanf("%lld",&a)
#define sff(a,b)         scanf("%d %d",&a,&b)
#define sffl(a,b)        scanf("%lld %lld",&a,&b)
#define sfff(a,b,c)      scanf("%d %d %d",&a,&b,&c)
#define sfffl(a,b,c)     scanf("%lld %lld %lld",&a,&b,&c)
#define loop(i,n)        for(int i=0;i<n;i++)
#define loop1(i,n)       for(int i=1;i<=n;i++)
#define REP(i,a,b)       for(int i=a;i<b;i++)
#define RREP(i,a,b)      for(int i=a;i>=b;i--)
#define intlim           2147483648
#define ull              unsigned long long
// #define gcd(a, b)        __gcd(a, b)
// #define lcm(a, b)        ((a)*((b)/gcd(a,b)))
#define mk(x1,y1) make_pair(x1, y1)
#define maxl 10000
#define psz 20
#define Fin      freopen("input.txt","r",stdin)
#define Fout     freopen("output.txt","w",stdout)


/*----------------------Graph Moves----------------*/
const int fx[]={+1,-1,+0,+0};
const int fy[]={+0,+0,+1,-1};
//const int fx[]={+0,+0,+1,-1,-1,+1,-1,+1};   // Kings Move
//const int fy[]={-1,+1,+0,+0,+1,+1,-1,-1};  // Kings Move
//const int fx[]={-2, -2, -1, -1,  1,  1,  2,  2};  // Knights Move
//const int fy[]={-1,  1, -2,  2, -2,  2, -1,  1}; // Knights Move
/*------------------------------------------------*/
// freopen("in.txt", "r", stdin);
    // ios::sync_with_stdio(0);
    // cin.tie(0);

/*-----------------------Bitmask------------------*/
//int Set(int N,int pos){return N=N | (1<<pos);}
//int reset(int N,int pos){return N= N & ~(1<<pos);}

//bool check(int N,int pos){return (bool)(N & (1<<pos));}
//freopen("in.txt","r",stdin);
//freopen("out.txt","w",stdout);
//ios_base::sync_with_stdio(false);
//cin.tie(NULL);
// ios::sync_with_stdio(0);
// cin.tie(0);
using namespace std;

vector<int>v;
int n, m;
vector<int>g[maxl + 2];
int inDegree[maxl + 2];
int lebel[maxl + 2];
int maxLabel = 0;
int position[maxl + 2];

bool TopoLogicalSort()
{
    queue<int>q;
    vector<int>afterSort;

    for(int i = 0; i < int( v.size() ); i++)
    {
        int indegreeZero = v[i];
        if(inDegree[indegreeZero] != 0){
        	continue;
        }
        q.push(indegreeZero);
        afterSort.pb(indegreeZero);
        lebel[indegreeZero] = 0;
    }

    int cnt=0;
    while( !q.empty() )
    {
        int u = q.front();
        cnt++;
        q.pop();
       // cout<<"U  "<<u << endl;
        for(int i = 0; i < int( g[u].size() ); i++)
        {
            int v = g[u][i];
            if(--inDegree[v] == 0){
            	q.push(v);
           // 	cout<<"C  "<<v<<endl;
            	lebel[v] = lebel[u] + 1;
            } 
        }
    }
   // cout<<"T  "<<cnt <<"  "<<n<<endl;
    if( cnt != n) return false; // if traversing nodes is not equal to total nodes, means not possible.
 
    return true;
}


int main(){

    ios_base::sync_with_stdio(false); cin.tie(0);

  	cin >> n >> m;

  	int serial[n + 2];

  	for(int i = 0; i < m; i++){
  		int u, v;
  		cin >> u >> v;
  		g[u].pb(v);
  		g[v].pb(u);
  		inDegree[v]++;
  	}

  	for(int i = 1; i <= n; i++){
  		if(inDegree[i] == 0){
  			v.pb(i);
  		}
  	}
  	

  	int ans = true;
  	if(TopoLogicalSort()){
  		cout<< "YES" << endl;
  	}else{
  		cout << "NO" << endl;
  	}
    return 0;
}