CF #405 (Div. 2) B. Bear ad Friendship Condition (dfs+完全图)

#include <bits/stdc++.h>
#define maxn 150000
#define ll long long
using namespace std;

vector <int> k[maxn+5];
bool visit[maxn+5];
int t;

void dfs(int x,int &v,int &e)
{
    assert(!visit[x]);
    v++;
    visit[x]=true;
    e+=k[x].size();
    for(int i=0;i<k[x].size();i++)
    {
        if(visit[k[x][i]]==0)
        dfs(k[x][i],v,e);
    }
}

int main()
{
    int n,m;
    while(cin>>n>>m)
    {
        for(int i=0;i<m;i++)
        {
            int x,y;
            scanf("%d %d",&x,&y);
            k[x].push_back(y);
            k[y].push_back(x);
        }
        for(int i=1;i<=n;i++)
        {
            if(!visit[i])
            {
                int v=0,e=0;
                dfs(i,v,e);
                if(e!=(long long) v*(v-1))
                {
                    cout<<"NO"<<endl;
                    return 0;
                }
            }
        }
        cout<<"YES"<<endl;
    }
    return 0;
}