reorg and add code for second edition

10_greedy_algorithms/swift/01_set_covering.swift

@@ -0,0 +1,35 @@
+import Foundation
+
+// You pass an array in, and it gets converted to a set.
+var statesNeeded : Set = ["mt", "wa", "or", "id", "nv", "ut", "ca", "az"]
+
+var stations = [String: Set<String>]()
+stations["kone"] = ["id", "nv", "ut"]
+stations["ktwo"] = ["wa", "id", "mt"]
+stations["kthree"] = ["or", "nv", "ca"]
+stations["kfour"] = ["nv", "ut"]
+stations["kfive"] = ["ca", "az"]
+
+var finalStations = Set<String>();
+
+while !statesNeeded.isEmpty {
+    var bestStation = String()
+    var statesCovered = Set<String>()
+    
+    for station in stations {
+        var covered = statesNeeded.intersection(station.value)
+        if covered.count > statesCovered.count {
+            bestStation = station.key
+            statesCovered = covered
+        }
+        statesNeeded = statesNeeded.subtracting(statesCovered)
+        //Swift note: We should avoid adding empty station to Set
+        if !bestStation.isEmpty {
+            finalStations.insert(bestStation)
+        }
+    }
+}
+
+print(finalStations) // -> ["kone", "kfive", "ktwo", "kthree"]
+
+