java dijkstra improvements

Author: Morfly

README.md

@@ -11,7 +11,7 @@ An encyclopedia of computer science algorithms and data structures.
 ## Algorithms
 ...
 
-All the links refer to `Kotlin` implementations by default. If you prefer another language feel free to check the list of [available languages](#available-languages).
+By default all the links below refer to `Kotlin` algorithm implementations. If you prefer another language feel free to check the list of [available languages](#available-languages).
 
 `B` - Base, `A` - Advanced
 ### By category

src/java/sequential/graph/dijkstra/Dijkstra.java

@@ -7,14 +7,16 @@
 import java.util.Map;
 import java.util.Set;
 
+import static java.util.Map.Entry.comparingByValue;
 
-class Dijkstra<T> {
+
+class Dijkstra {
 
     private static final double INFINITY = Double.POSITIVE_INFINITY;
 
-    
+
     public static <T> List<T> dijkstra(Map<T, Map<T, Double>> graph, T root, T target) {
-        if(!graph.containsKey(root)) throw new IllegalArgumentException("Invalid root.");
+        if (!graph.containsKey(root)) throw new IllegalArgumentException("Invalid root.");
 
         var costs = new HashMap<>(graph.get(root));
         var explored = new HashSet<T>();
@@ -43,16 +45,13 @@ public static <T> List<T> dijkstra(Map<T, Map<T, Double>> graph, T root, T targe
     }
 
     private static <T> T findMinCostNode(Map<T, Double> costs, Set<T> explored) {
-        var minCost = INFINITY;
-        T minCostNode = null;
-        for (var node : costs.keySet()) {
-            var cost = costs.get(node);
-            if (!explored.contains(node) && cost < minCost) {
-                minCost = cost;
-                minCostNode = node;
-            }
-        }
-        return minCostNode;
+        return costs.entrySet().stream()
+                // ignoring already explored nodes.
+                .filter(node -> !explored.contains(node.getKey()))
+                // finding the min cost node. `value` corresponds to node cost.
+                .min(comparingByValue())
+                // returning node object or `null` if the min node was not found.
+                .map(node -> node.getKey()).orElse(null);
     }
 
     private static <T> List<T> buildPathToNode(T target, Map<T, T> parents) {

src/kotlin/sequential/graph/dijkstra/Dijkstra.kt

@@ -16,7 +16,7 @@ private val INFINITY = Double.POSITIVE_INFINITY
 /**
  * Finds shortest path to the [target] node using Dijkstra algorithm.
  */
-fun <T> Graph<T>.dijkstra(root: T, target: T): Collection<T> {
+fun <T: Any> Graph<T>.dijkstra(root: T, target: T): Collection<T> {
     val graph = this
     require(root in graph) { "Invalid root." }
 
@@ -55,16 +55,12 @@ private fun <T> Map<T, Double>.findMinCostNode(explored: Collection<T>): T? =
 /**
  * Build path from root to target node.
  */
-private fun <T> buildPathToNode(target: T, parents: Map<T, T>): List<T> {
+private fun <T: Any> buildPathToNode(target: T, parents: Map<T, T>): List<T> {
     require(target in parents) { "Invalid target."}
 
-    val path = mutableListOf<T>()
-    var node: T? = target
-    while (node != null) {
-        path += node
-        node = parents[node]
-    }
-    return path.reversed()
+    return generateSequence(target) { node -> parents[node] }
+        .toList()
+        .asReversed()
 }
 
 

src/swift/sequential/graph/bfs/BreadthFirstTraversal.swift

@@ -7,7 +7,7 @@ func bfs<T>(in graph: Graph<T>, withRoot root: T) -> [T] {
     var searchQueue = [root]
     var queuePopIndex = 0
 
-    while searchQueue.count > queuePopIndex {
+    while queuePopIndex < searchQueue.count {
         let node = searchQueue[queuePopIndex]
         queuePopIndex += 1
 

src/swift/sequential/graph/dijkstra/Dijkstra.swift

@@ -55,7 +55,7 @@ private func findMinCostNode<T>(in costs: [T: Double], except explored: Set<T>)
         .filter { node, cost in !explored.contains(node) }
         // finding the min cost node. `value` corresponds to node cost.
         .min { cost1, cost2 in cost1.value < cost2.value}?
-        // returning node object or `nil` if the node was not found.
+        // returning node object or `nil` if the min node was not found.
         .key
 }