Refactor BipartiteGraphDFS.java, add Junit tests (#5606)

* Refactor `BipartiteGraphDFS.java`, add Junit tests

* Update directory

* Fix

* Add suggested changes

* Update BipartiteGraphDFS.java

---------

Co-authored-by: Hardvan <Hardvan@users.noreply.github.com>
Co-authored-by: Alex Klymenko <alexanderklmn@gmail.com>

DIRECTORY.md

@@ -676,6 +676,7 @@
             * dynamicarray
               * [DynamicArrayTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/dynamicarray/DynamicArrayTest.java)
             * graphs
+              * [BipartiteGraphDFSTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/graphs/BipartiteGraphDFSTest.java)
               * [BoruvkaAlgorithmTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/graphs/BoruvkaAlgorithmTest.java)
               * [DijkstraAlgorithmTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/graphs/DijkstraAlgorithmTest.java)
               * [EdmondsBlossomAlgorithmTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/graphs/EdmondsBlossomAlgorithmTest.java)

src/main/java/com/thealgorithms/datastructures/graphs/BipartiteGraphDFS.java

@@ -1,23 +1,49 @@
 package com.thealgorithms.datastructures.graphs;
 
-import java.io.BufferedReader;
-import java.io.IOException;
-import java.io.InputStreamReader;
 import java.util.ArrayList;
 import java.util.Arrays;
 
 /**
- * Given an adjacency list of a graph adj of V no. of vertices having 0 based
+ * This class provides a method to check if a given undirected graph is bipartite using Depth-First Search (DFS).
- * index. Check whether the graph is bipartite or not.
+ * A bipartite graph is a graph whose vertices can be divided into two disjoint sets such that no two vertices
+ * within the same set are adjacent. In other words, all edges must go between the two sets.
  *
- * Input : {{0, 1, 0, 1}, {1, 0, 1, 0}, {0, 1, 0, 1}, {1, 0, 1, 0}}
+ * The implementation leverages DFS to attempt to color the graph using two colors. If we can color the graph such
+ * that no two adjacent vertices have the same color, the graph is bipartite.
  *
- * Output : YES
+ * Example:
+ * Input (Adjacency Matrix):
+ * {{0, 1, 0, 1},
+ *  {1, 0, 1, 0},
+ *  {0, 1, 0, 1},
+ *  {1, 0, 1, 0}}
+ *
+ * Output: YES (This graph is bipartite)
+ *
+ * Input (Adjacency Matrix):
+ * {{0, 1, 1, 0},
+ *  {1, 0, 1, 0},
+ *  {1, 1, 0, 1},
+ *  {0, 0, 1, 0}}
+ *
+ * Output: NO (This graph is not bipartite)
  */
 public final class BipartiteGraphDFS {
     private BipartiteGraphDFS() {
     }
 
+    /**
+     * Helper method to perform DFS and check if the graph is bipartite.
+     *
+     * During DFS traversal, this method attempts to color each vertex in such a way
+     * that no two adjacent vertices share the same color.
+     *
+     * @param v     Number of vertices in the graph
+     * @param adj   Adjacency list of the graph where each index i contains a list of adjacent vertices
+     * @param color Array to store the color assigned to each vertex (-1 indicates uncolored)
+     * @param node  Current vertex being processed
+     * @return      True if the graph (or component of the graph) is bipartite, otherwise false
+     */
     private static boolean bipartite(int v, ArrayList<ArrayList<Integer>> adj, int[] color, int node) {
         if (color[node] == -1) {
             color[node] = 1;
@@ -35,11 +61,16 @@ public final class BipartiteGraphDFS {
         return true;
     }
 
+    /**
+     * Method to check if the graph is bipartite.
+     *
+     * @param v   Number of vertices in the graph
+     * @param adj Adjacency list of the graph
+     * @return    True if the graph is bipartite, otherwise false
+     */
     public static boolean isBipartite(int v, ArrayList<ArrayList<Integer>> adj) {
-        // Code here
         int[] color = new int[v + 1];
         Arrays.fill(color, -1);
-
         for (int i = 0; i < v; i++) {
             if (color[i] == -1) {
                 if (!bipartite(v, adj, color, i)) {
@@ -49,33 +80,4 @@ public final class BipartiteGraphDFS {
         }
         return true;
     }
-
-    public static void main(String[] args) throws IOException {
-        BufferedReader read = new BufferedReader(new InputStreamReader(System.in));
-        int t = Integer.parseInt(read.readLine().trim());
-        while (t-- > 0) {
-            String[] str1 = read.readLine().trim().split(" ");
-            int numVertices = Integer.parseInt(str1[0]);
-            int numEdges = Integer.parseInt(str1[1]);
-
-            ArrayList<ArrayList<Integer>> adj = new ArrayList<>();
-            for (int i = 0; i < numVertices; i++) {
-                adj.add(new ArrayList<>());
-            }
-            for (int i = 0; i < numEdges; i++) {
-                String[] str2 = read.readLine().trim().split(" ");
-                int vertexU = Integer.parseInt(str2[0]);
-                int vertexV = Integer.parseInt(str2[1]);
-                adj.get(vertexU).add(vertexV);
-                adj.get(vertexV).add(vertexU);
-            }
-
-            boolean ans = isBipartite(numVertices, adj);
-            if (ans) {
-                System.out.println("YES");
-            } else {
-                System.out.println("NO");
-            }
-        }
-    }
 }

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

@@ -0,0 +1,73 @@
+package com.thealgorithms.datastructures.graphs;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.ArrayList;
+import org.junit.jupiter.api.Test;
+
+public class BipartiteGraphDFSTest {
+
+    // Helper method to create an adjacency list from edges
+    private ArrayList<ArrayList<Integer>> createAdjacencyList(int numVertices, int[][] edges) {
+        ArrayList<ArrayList<Integer>> adj = new ArrayList<>();
+        for (int i = 0; i < numVertices; i++) {
+            adj.add(new ArrayList<>());
+        }
+        for (int[] edge : edges) {
+            int vertexU = edge[0];
+            int vertexV = edge[1];
+            adj.get(vertexU).add(vertexV);
+            adj.get(vertexV).add(vertexU);
+        }
+        return adj;
+    }
+
+    @Test
+    public void testBipartiteGraphEvenCycle() {
+        int numVertices = 4;
+        int[][] edges = {{0, 1}, {1, 2}, {2, 3}, {3, 0}}; // Even cycle
+        ArrayList<ArrayList<Integer>> adj = createAdjacencyList(numVertices, edges);
+        assertTrue(BipartiteGraphDFS.isBipartite(numVertices, adj), "Graph should be bipartite (even cycle)");
+    }
+
+    @Test
+    public void testBipartiteGraphOddCycle() {
+        int numVertices = 5;
+        int[][] edges = {{0, 1}, {1, 2}, {2, 0}, {1, 3}, {3, 4}}; // Odd cycle
+        ArrayList<ArrayList<Integer>> adj = createAdjacencyList(numVertices, edges);
+        assertFalse(BipartiteGraphDFS.isBipartite(numVertices, adj), "Graph should not be bipartite (odd cycle)");
+    }
+
+    @Test
+    public void testBipartiteGraphDisconnected() {
+        int numVertices = 6;
+        int[][] edges = {{0, 1}, {2, 3}, {4, 5}}; // Disconnected bipartite graphs
+        ArrayList<ArrayList<Integer>> adj = createAdjacencyList(numVertices, edges);
+        assertTrue(BipartiteGraphDFS.isBipartite(numVertices, adj), "Graph should be bipartite (disconnected)");
+    }
+
+    @Test
+    public void testBipartiteGraphSingleVertex() {
+        int numVertices = 1;
+        int[][] edges = {}; // Single vertex, no edges
+        ArrayList<ArrayList<Integer>> adj = createAdjacencyList(numVertices, edges);
+        assertTrue(BipartiteGraphDFS.isBipartite(numVertices, adj), "Graph should be bipartite (single vertex)");
+    }
+
+    @Test
+    public void testBipartiteGraphCompleteBipartite() {
+        int numVertices = 4;
+        int[][] edges = {{0, 2}, {0, 3}, {1, 2}, {1, 3}}; // K2,2 (Complete bipartite graph)
+        ArrayList<ArrayList<Integer>> adj = createAdjacencyList(numVertices, edges);
+        assertTrue(BipartiteGraphDFS.isBipartite(numVertices, adj), "Graph should be bipartite (complete bipartite)");
+    }
+
+    @Test
+    public void testBipartiteGraphNonBipartite() {
+        int numVertices = 3;
+        int[][] edges = {{0, 1}, {1, 2}, {2, 0}}; // Triangle (odd cycle)
+        ArrayList<ArrayList<Integer>> adj = createAdjacencyList(numVertices, edges);
+        assertFalse(BipartiteGraphDFS.isBipartite(numVertices, adj), "Graph should not be bipartite (triangle)");
+    }
+}