Improve robustness and test coverage for Kruskal MST

Author: anandraj095

src/main/java/com/thealgorithms/graph/KruskalMST.java

@@ -3,6 +3,7 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Objects;
 
 /**
  * Implementation of Kruskal's Algorithm to find
@@ -22,18 +23,22 @@ private KruskalMST() {
      * @param edges list of all edges in the graph
      * @return list of edges forming the MST
      * @throws IllegalArgumentException if vertices <= 0
+     * @throws NullPointerException if edges is null
      */
     public static List<Edge> findMST(final int vertices, final List<Edge> edges) {
         if (vertices <= 0) {
             throw new IllegalArgumentException("Number of vertices must be positive");
         }
 
+        Objects.requireNonNull(edges, "Edges list must not be null");
+
+        final List<Edge> sortedEdges = new ArrayList<>(edges);
+        Collections.sort(sortedEdges);
+
         final List<Edge> mst = new ArrayList<>();
         final DisjointSetUnion dsu = new DisjointSetUnion(vertices);
 
-        Collections.sort(edges);
-
-        for (final Edge edge : edges) {
+        for (final Edge edge : sortedEdges) {
             final int rootU = dsu.find(edge.source);
             final int rootV = dsu.find(edge.destination);
 

src/test/java/com/thealgorithms/graph/KruskalMSTTest.java

@@ -2,6 +2,7 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -47,4 +48,31 @@ void testFindMSTWithSingleVertex() {
         assertNotNull(mst, "MST should not be null");
         assertEquals(0, mst.size(), "MST of single vertex graph should be empty");
     }
+
+    @Test
+    void testInvalidVertexCountThrowsException() {
+        final List<Edge> edges = new ArrayList<>();
+
+        assertThrows(
+            IllegalArgumentException.class,
+            () -> KruskalMST.findMST(0, edges),
+            "Expected exception for non-positive vertex count"
+        );
+    }
+
+    @Test
+    void testPathCompressionScenario() {
+        final int vertices = 5;
+
+        final List<Edge> edges = new ArrayList<>();
+        edges.add(new Edge(0, 1, 1));
+        edges.add(new Edge(1, 2, 2));
+        edges.add(new Edge(2, 3, 3));
+        edges.add(new Edge(3, 4, 4));
+
+        final List<Edge> mst = KruskalMST.findMST(vertices, edges);
+
+        assertNotNull(mst);
+        assertEquals(vertices - 1, mst.size(), "MST should contain V-1 edges");
+    }
 }