https://stackoverflow.com/questions/48317842/group-items-in-a-list-based-on-two-properties-using-linq-mvc

stack.48317842.2.cs

// Same as initial recursive attempt but playing around with graph creation

void Main()
{
	var trip = new
	{
		Origin = "CMB",
		Destination = "DEL",
	};

	var data = new List<Segment>()
	{
		new Segment { Name = "01", Origin = "CMB", Destination = "MAA", Flight = "123"},
		new Segment { Name = "02", Origin = "MAA", Destination = "DEL", Flight = "543"},
		new Segment { Name = "03", Origin = "MAA", Destination = "DEL", Flight = "320"},
		new Segment { Name = "04", Origin = "CMB", Destination = "BOM", Flight = "644"},
		new Segment { Name = "05", Origin = "BOM", Destination = "DEL", Flight = "233"},
		new Segment { Name = "06", Origin = "CMB", Destination = "KMG", Flight = "233"},
		new Segment { Name = "07", Origin = "KMG", Destination = "PEK", Flight = "233"},
		new Segment { Name = "08", Origin = "PEK", Destination = "DEL", Flight = "233"},
		//new Segment { Name = "10", Origin = "KMG", Destination = "DEL", Flight = "233"},
		//new Segment { Name = "11", Origin = "CMB", Destination = "DEL", Flight = "111"},
	};
	data.Dump("Segments");
	
	var graph = new Analyzer(data).Graph();
	graph.Dump("Graph");

	var itineraries = graph.Traverse(p=>p.Children);
	itineraries.Dump("Itineraries");

	var query = from itinerary in itineraries
				let first = itinerary.First()
				let last = itinerary.Last()
				where 1 == 1
					&& trip.Origin == first.Origin
					&& trip.Destination == last.Destination
				group itinerary by first.Flight into groupings
				select groupings
					.SelectMany(p => p)
					.Distinct();

	query.Dump("Results");
}

public class Analyzer
{
	public Analyzer(IEnumerable<Segment> segments)
	{
		Segments = segments;
	}

	protected IEnumerable<Segment> Segments { get; set; }

	public IEnumerable<Node> Graph(IEnumerable<Segment> origins = null)
	{
		if (origins == null)
			origins = Segments;
		
		foreach (var origin in origins)
		{
			var destinations = Segments.Where(p => p.Origin == origin.Destination);
			yield return new Node(origin)
			{
				Children = Graph(destinations)
			};
		}
	}
}

public static class Extensions
{
	public static IEnumerable<IEnumerable<T>> Traverse<T>(
		this IEnumerable<T> subjects,
		Func<T, IEnumerable<T>> selector,
		Stack<T> current = null)
	{
		if (current == null)
			current = new Stack<T>();
			
		foreach (var subject in subjects)
		{
			current.Push(subject);
			var children = selector(subject);
			if (children.Any())
			{
				foreach (var child in children.Traverse(selector, current))
				{
					yield return child;
				}
			}
			else
			{
				yield return current.Reverse().ToList();
			}
			current.Pop();
		}
	}
}

public class Node : Segment
{
	public Node(Segment segment)
	{
		Name = segment.Name;
		Origin = segment.Origin;
		Destination = segment.Destination;
		Flight = segment.Flight;
	}
	
	public IEnumerable<Node> Children { get; set; }
	
	public string Next
	{
		get
		{
			return Children == null ? null : string.Join(",", Children.Select(p => p.Name));
		}
	}
	
	object ToDump() => new 
	{ 
		Name, 
		Origin, 
		Destination,
		Flight,
		Next 
	};
}

public class Segment
{
	public string Name { get; set; }
	public string Origin { get; set; }
	public string Destination { get; set; }
	public string Flight { get; set; }
}

stack.48317842.cs

void Main()
{
	var trip = new
	{
		Origin = "CMB",
		Destination = "DEL",
	};

	var segments = new List<Segment>()
	{
		new Segment { Name = "01", Origin = "CMB", Destination = "MAA", Flight = "123"},
		new Segment { Name = "02", Origin = "MAA", Destination = "DEL", Flight = "543"},
		new Segment { Name = "03", Origin = "MAA", Destination = "DEL", Flight = "320"},
		new Segment { Name = "04", Origin = "CMB", Destination = "BOM", Flight = "644"},
		new Segment { Name = "05", Origin = "BOM", Destination = "DEL", Flight = "233"},
		new Segment { Name = "06", Origin = "CMB", Destination = "KMG", Flight = "233"},
		new Segment { Name = "07", Origin = "KMG", Destination = "PEK", Flight = "233"},
		new Segment { Name = "08", Origin = "PEK", Destination = "DEL", Flight = "233"},
	};
	segments.Dump("Segments");
	
	var itineraries = new Analyzer(segments).Process();
	itineraries.Dump("Itineraries");

	var query = from itinerary in itineraries
				let first = itinerary.First()
				let last = itinerary.Last()
				where 1 == 1
					&& trip.Origin == first.Origin
					&& trip.Destination == last.Destination
				group itinerary by first.Flight into groupings
				select groupings
					.SelectMany(p => p)
					.Distinct();

	query.Dump("Results");
}

public class Analyzer
{
	public Analyzer(IEnumerable<Segment> segments)
	{
		Segments = segments;
	}

	protected IEnumerable<Segment> Segments { get; set; }

	public IEnumerable<IEnumerable<Segment>> Process(IEnumerable<Segment> origins = null, Stack<Segment> current = null)
	{
		if (origins == null)
			origins = Segments;
		if (current == null)
			current = new Stack<Segment>();

		foreach (var origin in origins)
		{
			current.Push(origin);

			var destinations = Segments.Where(p => p.Origin == origin.Destination);
			if (destinations.Any())
			{
				foreach (var child in Process(destinations, current))
				{
					yield return child;
				};
			}
			else
			{
				yield return current.Reverse().ToList();
			}

			current.Pop();
		}
	}
}

public class Segment
{
	public string Name { get; set; }
	public string Origin { get; set; }
	public string Destination { get; set; }
	public string Flight { get; set; }
}