Adding day 7

day7/part1.scala

@@ -0,0 +1,30 @@
+import scala.io.StdIn.readLine
+
+// "Step A must be finished before step B can begin."
+//  0    1 2    3  4        5      6    7 8   9
+val PREREQ_STEP_INDEX: Int = 1
+val STEP_INDEX: Int = 7
+
+val stepsWithParents = Iterator.continually(readLine)
+  .takeWhile(_ != null)
+  .map { l =>
+    val ws = l.split(" ")
+    ws(PREREQ_STEP_INDEX).head -> ws(STEP_INDEX).head }
+  .foldLeft(Map.empty[Char, Set[Char]]){
+    case (acc, (parent, child)) => acc +
+      (child -> (acc.getOrElse(child, Set.empty[Char]) + parent)) +
+      (parent -> (acc.getOrElse(parent, Set.empty[Char])))}
+  .toList
+
+def walkGraph(g: List[(Char, Set[Char])], result: String = ""): String = g match {
+  case Nil => result
+  case _ => 
+    val chosenParent = g.filter(_._2.isEmpty).map(_._1).min
+    walkGraph(
+      g
+        .filter(_._1 != chosenParent)
+        .map { case (k, v) => k -> v.filter(_ != chosenParent) },
+      result + chosenParent.toString)
+}
+
+println(walkGraph(stepsWithParents))