Added Greedy Set Cover algorithm (unweighted) as extension to Set.

Author: mrael2

SetCover.swift

@@ -0,0 +1,33 @@
+extension Set {
+    func cover(from array: Array<Set<Element>>) -> Array<Set<Element>>? {
+        var result: [Set<Element>]? = [Set<Element>]()
+        var remainingSet = self
+        
+        func largestIntersectingSet() -> Set<Element>? {
+            var largestIntersectionLength = 0
+            var largestSet: Set<Element>?
+            
+            for set in array {
+                let intersectionLength = remainingSet.intersect(set).count
+                if intersectionLength > largestIntersectionLength {
+                    largestIntersectionLength = intersectionLength
+                    largestSet = set
+                }
+            }
+            
+            return largestSet
+        }
+        
+        while remainingSet.count != 0 {
+            guard let largestSet = largestIntersectingSet() else { break }
+            result!.append(largestSet)
+            remainingSet = remainingSet.subtract(largestSet)
+        }
+        
+        if remainingSet.count != 0 || self.count == 0 {
+            result = nil
+        }
+        
+        return result
+    }
+}