Дерево

#define _CRT_SECURE_NO_WARNINS
#include <fstream>
#include <vector>
#include <string>
#include <set>
#include <algorithm>
#include <map>

using namespace std;
ifstream fin("input.txt");
ofstream fout("output.txt");
vector <vector<int>> mat;
int vertex, edges; //вершина, грань
vector <int> used(vertex);
//vector <int> Pr(vertex, 0);

bool dfs(int v, int prev){
    used[v] = 1;
    for (int i = 0; i < mat[v].size(); i++)
    {
        if (used[i] == 0 && mat[v][i] == 1 && i != prev){
            dfs(i, v);
            used[v] = 2;
        }
        if (used[i] == 1 && i != prev && mat[v][i] == 1)
            return false;
    }
    used[v] = 2;
    return true;

}


int main(){

fin >> vertex >> edges;
    used.resize(vertex);
    mat.resize(vertex, vector<int>(vertex));

    for (int i = 0; i < vertex; i++)
    {
        for (int j=0; j < vertex; j++){
            mat[i][j] = 0;
        }
    }

    for (int i = 0; i < used.size(); i++)
    {
        used[i] = 0;

    }

    for (int i = 0; i < edges; i++){
        int a, b;
        fin >> a >> b;
        a--;
        b--;
        mat[a][b] = 1;
        mat[b][a] = 1;
    }
    int qq = 0;
if (dfs(0, 0) == false){
        qq =0;
    }
    else{
        qq = 1;
    }
    for (int i = 0; i < used.size(); i++){
        if (used[i] != 2){
            qq = 0;
        }
    }
    if (qq == 1){
            fout << "YES";
    }
    else{
        fout << "NO";
    }

    return 0;

}