Enhance docs, add tests in KahnsAlgorithm (#5965)

src/test/java/com/thealgorithms/datastructures/graphs/KahnsAlgorithmTest.java

@@ -0,0 +1,77 @@
+package com.thealgorithms.datastructures.graphs;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.ArrayList;
+import org.junit.jupiter.api.Test;
+
+class KahnsAlgorithmTest {
+
+    @Test
+    void testBasicGraph() {
+        // Test case with a basic directed acyclic graph (DAG)
+        AdjacencyList<String> graph = new AdjacencyList<>();
+        graph.addEdge("a", "b");
+        graph.addEdge("c", "a");
+        graph.addEdge("a", "d");
+        graph.addEdge("b", "d");
+
+        TopologicalSort<String> topSort = new TopologicalSort<>(graph);
+        ArrayList<String> result = topSort.topSortOrder();
+
+        String[] expectedOrder = {"c", "a", "b", "d"};
+        assertArrayEquals(expectedOrder, result.toArray());
+    }
+
+    @Test
+    void testGraphWithMultipleSources() {
+        // Test case where graph has multiple independent sources
+        AdjacencyList<String> graph = new AdjacencyList<>();
+        graph.addEdge("a", "c");
+        graph.addEdge("b", "c");
+
+        TopologicalSort<String> topSort = new TopologicalSort<>(graph);
+        ArrayList<String> result = topSort.topSortOrder();
+
+        String[] expectedOrder = {"a", "b", "c"};
+        assertArrayEquals(expectedOrder, result.toArray());
+    }
+
+    @Test
+    void testDisconnectedGraph() {
+        // Test case for disconnected graph
+        AdjacencyList<String> graph = new AdjacencyList<>();
+        graph.addEdge("a", "b");
+        graph.addEdge("c", "d");
+
+        TopologicalSort<String> topSort = new TopologicalSort<>(graph);
+        ArrayList<String> result = topSort.topSortOrder();
+
+        String[] expectedOrder = {"a", "c", "b", "d"};
+        assertArrayEquals(expectedOrder, result.toArray());
+    }
+
+    @Test
+    void testGraphWithCycle() {
+        // Test case for a graph with a cycle - topological sorting is not possible
+        AdjacencyList<String> graph = new AdjacencyList<>();
+        graph.addEdge("a", "b");
+        graph.addEdge("b", "c");
+        graph.addEdge("c", "a");
+
+        TopologicalSort<String> topSort = new TopologicalSort<>(graph);
+
+        assertThrows(IllegalStateException.class, () -> topSort.topSortOrder());
+    }
+
+    @Test
+    void testSingleNodeGraph() {
+        AdjacencyList<String> graph = new AdjacencyList<>();
+        graph.addEdge("a", "a"); // self-loop
+
+        TopologicalSort<String> topSort = new TopologicalSort<>(graph);
+
+        assertThrows(IllegalStateException.class, () -> topSort.topSortOrder());
+    }
+}