Add docs to portals.

src/School/Portals.cs

@@ -35,40 +35,50 @@ namespace CodingInterview.School
 		/// Returns the minmum amount of money needed to get all plannets connected.
 		/// </summary>
 		/// <param name="costs">the costs of building a portal on each planet (index is a planet)</param>
-		/// <param name="connections">pairs of indices representing planets which already have portals between them</param>
+		/// <param name="connections">pairs of indices representing planets which already have portals between them (here indexaton starts from 1)</param>
 		/// <returns>the minmum amount of money needed to get all plannets connected</returns>
 		public int Solve(int[] costs, (int from, int to)[] connections)
 		{
+			// create planets given costs
 			var planets = new Planet[costs.Length];
 			for (var planet = 0; planet < costs.Length; planet++)
 			{
 				planets[planet] = new Planet { Cost = costs[planet] };
 			}
+			// setup connections
 			foreach (var connection in connections)
 			{
 				planets[connection.from - 1].Neighbors.Add(planets[connection.to - 1]);
 				planets[connection.to - 1].Neighbors.Add(planets[connection.from - 1]);
 			}
 
+			// go over all planets in a BFS manner
 			var minCosts = new List<int>();
 			for (var planet = 0; planet < costs.Length; planet++)
 			{
+				// skip visited planets
 				if (planets[planet].Visited)
 				{
 					continue;
 				}
 
+				// initialize BFS queue with the initial (unvisited) planet
 				var minCost = Int32.MaxValue;
 				var bfsQueue = new Queue<Planet>();
 				bfsQueue.Enqueue(planets[planet]);
 
+				// until entire cluster is traversed
 				while (bfsQueue.Count > 0)
 				{
 					var current = bfsQueue.Dequeue();
+					// skip visited
 					if (!current.Visited)
 					{
+						// update min cost in the cluster
 						minCost = Math.Min(current.Cost, minCost);
+						// don't go here again
 						current.Visited = true;
+						// add not visited neighbors to the queue
 						foreach (var neighbor in current.Neighbors)
 						{
 							if (!neighbor.Visited)
@@ -78,14 +88,16 @@ namespace CodingInterview.School
 						}
 					}
 				}
-				if (minCost != Int32.MaxValue)
-				{
+
+				// remember min cost of the cluster
 				minCosts.Add(minCost);
 			}
-			}
 
+			// find the smallest of all min costs
 			var minOfMin = minCosts.Min();
 
+			// if T is the number of clusters, the final cost is building T - 1 portals in the cheapest cluster on its cheapest planet,
+			// then building a portal on a cheapest planet on each of the other clusters
 			return minOfMin * (minCosts.Count - 1) + (minCosts.Sum() - minOfMin);
 		}
 	}