programming-examples/c-sharp/Strings/AdjacencyList.cs

using System;
using System.Collections.Generic;
using System.Diagnostics.Contracts;
using System.Linq;
using System.Text;

//TODO: unweighted adjacency list and weighted adjacency list
namespace DataStructures.AdjacencyList
{
    [Serializable]
    public class AdjacencyList<T>
    {
        private readonly Dictionary<T, HashSet<T>> dict;

        public IList<T> Vertices 
        {
            get { return dict.Keys.ToList(); }
        }

        public int Count 
        {
            get { return dict.Count; }
        }

        public AdjacencyList() 
        {
            dict = new Dictionary<T, HashSet<T>>();
        }

        public AdjacencyList(int capacity)
        {
            Contract.Requires<ArgumentOutOfRangeException>(capacity > 0);
            
            dict = new Dictionary<T, HashSet<T>>(capacity);
        }

        public void AddVertex(T vertex)
        {
            Contract.Requires<ArgumentNullException>(vertex != null);
            
            if(dict.ContainsKey(vertex))
            {
                return;
            }
            dict[vertex] = new HashSet<T>();
        }

        public void AddEdge(T vertex1, T vertex2) 
        {
            Contract.Requires<ArgumentNullException>(vertex1 != null);
            Contract.Requires<ArgumentNullException>(vertex2 != null);

            if (!dict.ContainsKey(vertex1))
            {
                dict[vertex1] = new HashSet<T>();
            }
            if (!dict.ContainsKey(vertex2))
            {
                dict[vertex2] = new HashSet<T>();
            }
            dict[vertex1].Add(vertex2);
            dict[vertex2].Add(vertex1);
        }

        public bool IsNeighbourOf(T vertex, T neighbour) 
        {
            Contract.Requires<ArgumentNullException>(vertex != null);
            Contract.Requires<ArgumentNullException>(neighbour != null);

            return dict.ContainsKey(vertex) && dict[vertex].Contains(neighbour);
        }

        public IList<T> GetNeighbours(T vertex) 
        {
            Contract.Requires<ArgumentNullException>(vertex != null);
            
            return dict.ContainsKey(vertex)? dict[vertex].ToList(): new List<T>();
        } 

        public IList<T> this[T vertex]
        {
            get 
            {
                return GetNeighbours(vertex);
            }
            set 
            {
                Contract.Requires<ArgumentNullException>(vertex != null);

                dict[vertex] = new HashSet<T>(value);
            }
        }

        public IList<Tuple<T, T>> GetEdgeList()
        {
            var list = new List<Tuple<T, T>>();

            foreach (var entry in dict)
            {
                var key = entry.Key;
                var hashSet = entry.Value;
                list.AddRange(hashSet.Select(hashEntry => new Tuple<T, T>(key, hashEntry)));
            }
            return list;
        }
    }
}