Dijkstra via Priority Queue

Author: JakubSchwenkbeck

src/main/scala/algorithms/graph/Dijkstra.scala

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+package algorithms.graph
+
+import datastructures.graph.Graph
+import datastructures.basic.PriorityQueue
+import scala.collection.mutable
+
+/// Dijkstra's Algorithm
+/// Finds the shortest paths from a starting node to all other nodes in a weighted graph
+/// Uses a priority queue-based approach
+/// Runs in O((V + E) log V) time complexity
+
+def dijkstra[T](graph: Graph[T], start: T): Unit = {
+  val distances = mutable.Map[T, Int]().withDefaultValue(Int.MaxValue)
+  val visited = mutable.Set[T]()
+  val priorityQueue = PriorityQueue[(T, Int)]()(Ordering.by(-_._2))
+
+  distances(start) = 0
+  priorityQueue.enqueue((start, 0))
+  while (!priorityQueue.isEmpty) {
+    val (currentNode: T, currentDistance: Int) = priorityQueue.dequeue().get // unsafe getter for now
+    if (!visited.contains(currentNode)) {
+      visited.add(currentNode)
+
+      for ((neighbor, weight) <- graph.getNeighbors(currentNode)) {
+        if (!visited.contains(neighbor)) {
+          val newDistance = currentDistance + weight
+          if (newDistance < distances(neighbor)) {
+            distances(neighbor) = newDistance.toInt
+            priorityQueue.enqueue((neighbor, newDistance.toInt))
+          }
+        }
+      }
+    }
+  }
+
+  distances.toMap
+
+}