Add TopologicalSortDFSTest with JUnit 5 tests

Author: SiddhiPandey08

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

@@ -0,0 +1,148 @@
+package com.thealgorithms.datastructures.graphs;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+/**
+ * JUnit 5 tests for {@link TopologicalSortDFS}.
+ *
+ * Test cases cover:
+ * - Standard DAG with a clear expected topological order
+ * - Multiple valid orderings (validated by constraint checking)
+ * - Graphs with cycles (must throw IllegalStateException)
+ * - Single-node graph
+ * - Disconnected graph
+ */
+class TopologicalSortDFSTest {
+
+    /**
+     * Checks that every edge u → v satisfies: u appears before v in the result.
+     */
+    private static void assertValidTopologicalOrder(List<Integer> order, int[][] edges) {
+        for (int[] edge : edges) {
+            int u = edge[0];
+            int v = edge[1];
+            assertTrue(
+                    order.indexOf(u) < order.indexOf(v),
+                    "Expected " + u + " to appear before " + v + " in topological order. Got: " + order);
+        }
+    }
+
+    @Test
+    void testSimpleLinearGraph() {
+        // 0 → 1 → 2 → 3
+        TopologicalSortDFS graph = new TopologicalSortDFS();
+        graph.addEdge(0, 1);
+        graph.addEdge(1, 2);
+        graph.addEdge(2, 3);
+
+        List<Integer> result = graph.topologicalSort();
+        assertEquals(List.of(0, 1, 2, 3), result);
+    }
+
+    @Test
+    void testDAGWithMultiplePaths() {
+        // Graph:
+        // 5 → 2
+        // 5 → 0
+        // 4 → 0
+        // 4 → 1
+        // 2 → 3
+        // 3 → 1
+        TopologicalSortDFS graph = new TopologicalSortDFS();
+        int[][] edges = { { 5, 2 }, { 5, 0 }, { 4, 0 }, { 4, 1 }, { 2, 3 }, { 3, 1 } };
+        for (int[] edge : edges) {
+            graph.addEdge(edge[0], edge[1]);
+        }
+
+        List<Integer> result = graph.topologicalSort();
+        assertEquals(6, result.size());
+        assertValidTopologicalOrder(result, edges);
+    }
+
+    @Test
+    void testBuildOrderDAG() {
+        // Simulates build dependency: A depends on nothing, B depends on A, C depends
+        // on A and B
+        // A=0, B=1, C=2, D=3
+        // 0 → 1, 0 → 2, 1 → 3, 2 → 3
+        TopologicalSortDFS graph = new TopologicalSortDFS();
+        int[][] edges = { { 0, 1 }, { 0, 2 }, { 1, 3 }, { 2, 3 } };
+        for (int[] edge : edges) {
+            graph.addEdge(edge[0], edge[1]);
+        }
+
+        List<Integer> result = graph.topologicalSort();
+        assertEquals(4, result.size());
+        assertValidTopologicalOrder(result, edges);
+    }
+
+    @Test
+    void testSingleVertex() {
+        TopologicalSortDFS graph = new TopologicalSortDFS();
+        graph.addVertex(42);
+
+        List<Integer> result = graph.topologicalSort();
+        assertEquals(List.of(42), result);
+    }
+
+    @Test
+    void testDisconnectedGraph() {
+        // Two disconnected components: 0 → 1 and 2 → 3
+        TopologicalSortDFS graph = new TopologicalSortDFS();
+        int[][] edges = { { 0, 1 }, { 2, 3 } };
+        for (int[] edge : edges) {
+            graph.addEdge(edge[0], edge[1]);
+        }
+
+        List<Integer> result = graph.topologicalSort();
+        assertEquals(4, result.size());
+        assertValidTopologicalOrder(result, edges);
+    }
+
+    @Test
+    void testSimpleCycleThrowsException() {
+        // 0 → 1 → 2 → 0 (cycle)
+        TopologicalSortDFS graph = new TopologicalSortDFS();
+        graph.addEdge(0, 1);
+        graph.addEdge(1, 2);
+        graph.addEdge(2, 0);
+
+        assertThrows(
+                IllegalStateException.class,
+                graph::topologicalSort,
+                "Expected IllegalStateException for cyclic graph");
+    }
+
+    @Test
+    void testSelfLoopThrowsException() {
+        // A self-loop is a trivial cycle: 0 → 0
+        TopologicalSortDFS graph = new TopologicalSortDFS();
+        graph.addEdge(0, 0);
+
+        assertThrows(
+                IllegalStateException.class,
+                graph::topologicalSort,
+                "Expected IllegalStateException for self-loop");
+    }
+
+    @Test
+    void testLargerCycleThrowsException() {
+        // 0 → 1 → 2 → 3 → 4 → 2 (cycle at 2 → 3 → 4 → 2)
+        TopologicalSortDFS graph = new TopologicalSortDFS();
+        graph.addEdge(0, 1);
+        graph.addEdge(1, 2);
+        graph.addEdge(2, 3);
+        graph.addEdge(3, 4);
+        graph.addEdge(4, 2);
+
+        assertThrows(
+                IllegalStateException.class,
+                graph::topologicalSort,
+                "Expected IllegalStateException for graph with cycle");
+    }
+}