Leetcode 547 - friends circle - study code

Leetcode547_friendsCircle_StudyCode.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Leetcode547_friendsCircle_DFSSolution
{
    /// <summary>
    /// Leetcode 547
    /// source code from C++ code: 
    /// https://discuss.leetcode.com/topic/85032/20-line-c-dfs-solution
    /// Learn unordered_map, unordered_set
    /// </summary>
    class Program
    {
        static void Main(string[] args)
        {
            RunTestcase(); 
        }

        public static void RunTestcase()
        {
            var M = new int[,] {{1,0,0},{0,1,0},{0,0,1} }; 
            var result = FindCircleNum(M); 
        }

        // the friendship graph
        static Dictionary<int, HashSet<int>> friends = new Dictionary<int,HashSet<int>>(); 
        static HashSet<int> visited = new HashSet<int>();

        public static int FindCircleNum(int[,] M) {
            friends.Clear();
            visited.Clear(); 

            int numberOfCircles=0;
            for (int i = 0; i < M.GetLength(0); i++) // build the graph
            {
                for (int j = 0; j < M.GetLength(1); j++)
                {
                    if (i != j && M[i, j] == 1)
                    {
                        if (friends.ContainsKey(i))
                        {
                            friends[i].Add(j);
                        }
                        else
                        {
                            var set = new HashSet<int>();
                            set.Add(j);
                            friends.Add(i, set); 
                        }
                    }
                }
            }

            // explore the graph to see how many isolated graphs
            for (int i = 0; i < M.GetLength(0); i++) 
            {
                if (!visited.Contains(i))
                {
                    numberOfCircles++;
                    exploreFriends(i);
                }
            }

            return numberOfCircles;
        }
    
        /// <summary>
        /// recursive function - DFS
        /// </summary>
        /// <param name="i"></param>
        private static void exploreFriends(int i) { 
            visited.Add(i);
            if (!friends.ContainsKey(i))
            {
                return; 
            }

            foreach (var friend in friends[i])
            {
                if (!visited.Contains(friend))
                {
                    exploreFriends(friend);
                }
            }
        }
    }
}