reorg and add code for second edition

10_greedy_algorithms/PowerShell/01_set_covering.ps1

@@ -0,0 +1,42 @@
+# You pass an array in, and it gets converted to a set.
+$states_needed = New-Object System.Collections.Generic.HashSet[string] -ArgumentList `
+    (, [string[]]("mt", "wa", "or", "id", "nv", "ut", "ca", "az"))
+
+$stations = [ordered]@{}
+$stations["kone"] = New-Object System.Collections.Generic.HashSet[string] -ArgumentList `
+    (, [string[]]("id", "nv", "ut"))
+$stations["ktwo"] = New-Object System.Collections.Generic.HashSet[string] -ArgumentList `
+    (, [string[]]("wa", "id", "mt"))
+$stations["kthree"] = New-Object System.Collections.Generic.HashSet[string] -ArgumentList `
+    (, [string[]]("or", "nv", "ca"))
+$stations["kfour"] = New-Object System.Collections.Generic.HashSet[string] -ArgumentList `
+    (, [string[]]("nv", "ut"))
+$stations["kfive"] = New-Object System.Collections.Generic.HashSet[string] -ArgumentList `
+    (, [string[]]("ca", "az"))
+
+$final_stations = New-Object System.Collections.Generic.HashSet[string]
+
+while ($states_needed.Count -gt 0)
+{
+    $best_station = $null
+    $states_covered = New-Object System.Collections.Generic.HashSet[string]
+    foreach ($station in $stations.GetEnumerator()) {
+        $covered = New-Object System.Collections.Generic.HashSet[string]
+        # The .IntersectWith method deletes entries from the object so we need a temporary
+		# copy, should work in .NET 4.6+
+        $states_needed_inter = New-Object System.Collections.Generic.HashSet[string] $states_needed, $states_needed.Comparer
+        $states_needed_inter.IntersectWith($station.value)
+        $states_needed_inter | % {$covered.Add($_) >$null} 
+        if ($covered.count -gt $states_covered.count)
+        {
+            $best_station = $station.Key
+			# See the previous comment
+            $states_covered = New-Object System.Collections.Generic.HashSet[string] $covered, $covered.Comparer
+        }
+    }
+
+    $states_covered | % {$states_needed.Remove($_) >$null}
+    $final_stations.Add($best_station) >$null
+}
+
+Write-Host $final_stations