DFS topology sort

scala/src/main/scala/ch43_topology_sort/GraphTopology.scala

@@ -1,6 +1,7 @@
 package ch43_topology_sort
 
 import scala.collection.mutable
+import scala.collection.mutable.ArrayBuffer
 
 class GraphTopology(vertex: Int) {
 
@@ -16,7 +17,7 @@ class GraphTopology(vertex: Int) {
 
   def topologySortByKahn(): Array[Int] = {
     val seq = new mutable.ArrayBuffer[Int]()
-    //inDgrees contains all the inDegree for a given node
+    //inDegrees contains all the inDegree for a given node
     val inDegrees = new Array[Int](vertex)
     for (i <- Range(0, vertex)) {
       for (j <- adjacency(i).indices) {
@@ -53,6 +54,47 @@ class GraphTopology(vertex: Int) {
 
     seq.toArray
   }
+
+  def topologySortByDFS(): Array[Int] = {
+    val inverseAdj = new Array[mutable.MutableList[Int]](vertex)
+    for (i <- Range(0, vertex)) {
+      inverseAdj(i) = new mutable.MutableList[Int]()
+    }
+
+    //build the inverse adj
+    for (i <- Range(0, vertex)) {
+      for (j <- adjacency(i).indices) {
+        val index = adjacency(i).get(j).get
+        inverseAdj(index) += i
+      }
+    }
+
+    val visited = new Array[Boolean](vertex)
+    val seq = new ArrayBuffer[Int]()
+    for (i <- Range(0, vertex)) {
+      if (!visited(i)) {
+        visited(i) = true
+        //call dfs
+        seq ++= dfs(i, inverseAdj, visited)
+      }
+    }
+
+    seq.toArray
+  }
+
+  def dfs(index: Int, inverseAdj: Array[mutable.MutableList[Int]], visited: Array[Boolean]): ArrayBuffer[Int] = {
+    val seq = new ArrayBuffer[Int]()
+
+    for (i <- inverseAdj(index).indices) {
+      val sourceIndex = inverseAdj(index).get(i).get
+      if (!visited(sourceIndex)) {
+        visited(sourceIndex) = true
+        seq ++= dfs(sourceIndex, inverseAdj, visited)
+      }
+    }
+    seq += index
+    seq
+  }
 }
 
 

scala/src/test/scala/ch43_topology_sort/GraphTopologyTest.scala

@@ -61,4 +61,60 @@ class GraphTopologyTest extends FlatSpec with Matchers {
     val seq = graphTopology.topologySortByKahn()
     seq.map(nodes(_)).mkString(",") should equal("a")
   }
+
+  /*
+     a -> b
+     |    |
+    \|/  \|/
+     c -> d -> e
+   */
+  it should "topologySortByDFS - 1" in {
+    val nodes = Array("a", "b", "c", "d", "e")
+    val graphTopology = new GraphTopology(nodes.length)
+    graphTopology.addEdge(0, 1)
+    graphTopology.addEdge(0, 2)
+    graphTopology.addEdge(1, 3)
+    graphTopology.addEdge(2, 3)
+    graphTopology.addEdge(3, 4)
+
+    val seq = graphTopology.topologySortByDFS()
+    seq.map(nodes(_)).mkString(",") should equal("a,b,c,d,e")
+  }
+
+  /*
+     a -> d ---
+          |    |
+         \|/  \|/
+          e -> c
+   */
+  it should "topologySortByDFS - 2" in {
+    val nodes = Array("a", "b", "c", "d", "e")
+    val graphTopology = new GraphTopology(nodes.length)
+    graphTopology.addEdge(0, 3)
+    graphTopology.addEdge(3, 4)
+    graphTopology.addEdge(3, 2)
+    graphTopology.addEdge(4, 2)
+
+    val seq = graphTopology.topologySortByDFS()
+    seq.map(nodes(_)).mkString(",") should equal("a,b,d,e,c")
+  }
+
+  /*
+     a -> d <- b
+          |   /|\
+         \|/   |
+          e -> c
+   */
+  it should "topologySortByDFS - 3" in {
+    val nodes = Array("a", "b", "c", "d", "e")
+    val graphTopology = new GraphTopology(nodes.length)
+    graphTopology.addEdge(0, 3)
+    graphTopology.addEdge(3, 4)
+    graphTopology.addEdge(4, 2)
+    graphTopology.addEdge(2, 1)
+    graphTopology.addEdge(1, 3)
+
+    val seq = graphTopology.topologySortByKahn()
+    seq.map(nodes(_)).mkString(",") should equal("a")
+  }
 }