дерево

#include <iostream>
#include <string>
#include <sstream>
#include <cmath>
#include <algorithm>
#include <memory.h>
#include <stdio.h>
#include <vector>
#include <stack>
#include <deque>
#include <queue>
#include <vector>
#include <set>
#include <iterator>
#include <map>
#include <iomanip>
#define int long long
#define fir first
#define sec second
#define sp system("pause")
#define pb push_back
#define double long double
#define endl "\n"
#define un unsigned
using namespace std;

vector <int> mark(10000);
vector<set<int>> gr(10000);
int mas[105][105];
int ans = 0;
int kol_reb = 0;

void in_set(int n)
{
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            if (mas[i][j] == 1)
            {
                gr[i].insert(j);
                kol_reb++;
            }
        }
    }

    kol_reb /= 2;

    return;
}

void DFS_cycle(int v, int pr)
{
    mark[v] = 1;

    for (int i : gr[v])
    {
        if (mark[i] == 0)
        {
            DFS_cycle(i, v);
        }

        else if (i != pr)
        {
            ans++;
        }

    }

    return;
}

signed main()
{
    ios_base::sync_with_stdio();
    cin.tie(0);
    cout.tie(0);

    int n;
    cin >> n;

    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            cin >> mas[i][j];
        }
    }

    in_set(n);

    if (kol_reb != n - 1)
    {
        cout << "NO";

        sp;
        return 0;
    }

    for (int i = 1; i <= n; i++)
    {
        for (int i = 0; i <= n; i++)
        {
            mark[i] = 0;
        }

        DFS_cycle(i, -1);
    }

    if (ans != 0)
    {
        cout << "NO";
    }

    else
    {
        cout << "YES";
    }

    return 0;

}