Bear and Friendship Condition.cpp

#include<bits/stdc++.h>
 
using namespace std ;
 
typedef double D ;
typedef long long ll ;
typedef long double ld ;
typedef unsigned int ui ;
typedef unsigned long long ull ;
 
# define F first
# define S second
# define R return
# define C continue 
# define pb push_back 
# define pf push_front
# define mp make_pair
 
# define vi vector <int>
# define vb vector <bool>
# define vc vector <char>
# define vll vector <ll>
# define vs vector <string>
 
# define vvi vector < vector < int > >
# define vvb vector < vector < bool > >
# define vvc vector < vector < char > >
# define vvll vector < vector < ll > >
# define vvd vector < vector < D > >
# define vvld vector < vector < ld > >
 
# define pii pair < int , int >
# define pll pair < ll , ll >
# define pld pair < ld , ld >
# define pDD pair < D , D >
 
# define vpld vector < pld >
# define vpii vector < pii >
# define vpll vector < pll >
# define vpDD vector < pDD >
# define vvpii vector < vector < pii > >
 
# define all(v) (v).begin() , (v).end()
# define allrev(v) (v).rbegin() , (v).rend()
# define allcomp(v) v.begin() , v.end() , comp
# define allrevcomp(v) v.rbegin() , v.rend() , comp

# define dist(a,b,p,q) sqrt((p-a)*(p-a) + (q-b)*(q-b))

# define FAST ios_base :: sync_with_stdio (false) ; cin.tie(0) ; cout.tie(0)
 
const ll MOD = 1e9 + 7 ;
const int infi =  INT_MAX ;
const ll infll =  LLONG_MAX ;
const ld PI = 3.1415926535897932384626 ;

const int dx[] = {-1 , 0 , 1 , 0} ;
const int dy[] = {0 , 1 , 0 , -1} ;
 
///////////////////////////////////////////////////////////////////////////////////////

vb vis ;
vvi adj ;
int n , m , u , v , ed , nd ;

void dfs(int node) {
	vis[node] = true ;
	++nd ;
	for (int child : adj[node]) {
		++ed ;
		if (vis[child] == false) {
			dfs(child) ;
		}
	}
}

void solve (int test_case) {
	cin >> n >> m ;
	adj = vvi (n+1) ;
	vis = vb (n+1 , false) ;
	for (int i = 1 ; i <= m ; ++i) {
		cin >> u >> v ;
		adj[u].pb(v) ;
		adj[v].pb(u) ;
	}
	for (int i = 1 ; i <= n ; ++i) {
		if (vis[i] == false) {
			ed = nd = 0 ;
			dfs(i) ;
			ed /= 2 ;
			if (ed != (ll)nd*(nd-1)/2) {
				cout << "NO\n" ;
				return ;
			}
		}
	}
	cout << "YES\n" ;
}
 
int main(){
	//freopen ("input.txt","r",stdin) ;
	//freopen ("output.txt","w",stdout) ;
	FAST ;
	int tc = 1 ;
//	cin >> tc ;
	while (tc--){
		solve (tc) ;
	}
	return 0 ;
}