Add tests, remove main in UnionFind (#5678)

DIRECTORY.md

@@ -1019,6 +1019,7 @@
             * [SortOrderAgnosticBinarySearchTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/searches/SortOrderAgnosticBinarySearchTest.java)
             * [SquareRootBinarySearchTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/searches/SquareRootBinarySearchTest.java)
             * [TestSearchInARowAndColWiseSortedMatrix](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/searches/TestSearchInARowAndColWiseSortedMatrix.java)
+            * [UnionFindTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/searches/UnionFindTest.java)
           * sorts
             * [BeadSortTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/sorts/BeadSortTest.java)
             * [BinaryInsertionSortTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/sorts/BinaryInsertionSortTest.java)

src/main/java/com/thealgorithms/searches/UnionFind.java

@@ -4,11 +4,28 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 
+/**
+ * The Union-Find data structure, also known as Disjoint Set Union (DSU),
+ * is a data structure that tracks a set of elements partitioned into
+ * disjoint (non-overlapping) subsets. It supports two main operations:
+ *
+ * 1. **Find**: Determine which subset a particular element is in.
+ * 2. **Union**: Join two subsets into a single subset.
+ *
+ * This implementation uses path compression in the `find` operation
+ * and union by rank in the `union` operation for efficiency.
+ */
 public class UnionFind {
 
-    private final int[] p;
-    private final int[] r;
+    private final int[] p; // Parent array
+    private final int[] r; // Rank array
 
+    /**
+     * Initializes a Union-Find data structure with n elements.
+     * Each element is its own parent initially.
+     *
+     * @param n the number of elements
+     */
     public UnionFind(int n) {
         p = new int[n];
         r = new int[n];
@@ -18,6 +35,13 @@ public class UnionFind {
         }
     }
 
+    /**
+     * Finds the root of the set containing the element i.
+     * Uses path compression to flatten the structure.
+     *
+     * @param i the element to find
+     * @return the root of the set
+     */
     public int find(int i) {
         int parent = p[i];
 
@@ -25,12 +49,19 @@ public class UnionFind {
             return i;
         }
 
+        // Path compression
         final int result = find(parent);
         p[i] = result;
-
         return result;
     }
 
+    /**
+     * Unites the sets containing elements x and y.
+     * Uses union by rank to attach the smaller tree under the larger tree.
+     *
+     * @param x the first element
+     * @param y the second element
+     */
     public void union(int x, int y) {
         int r0 = find(x);
         int r1 = find(y);
@@ -39,6 +70,7 @@ public class UnionFind {
             return;
         }
 
+        // Union by rank
         if (r[r0] > r[r1]) {
             p[r1] = r0;
         } else if (r[r1] > r[r0]) {
@@ -49,39 +81,24 @@ public class UnionFind {
         }
     }
 
+    /**
+     * Counts the number of disjoint sets.
+     *
+     * @return the number of disjoint sets
+     */
     public int count() {
         List<Integer> parents = new ArrayList<>();
         for (int i = 0; i < p.length; i++) {
-            if (!parents.contains(find(i))) {
-                parents.add(find(i));
+            int root = find(i);
+            if (!parents.contains(root)) {
+                parents.add(root);
             }
         }
         return parents.size();
     }
 
+    @Override
     public String toString() {
         return "p " + Arrays.toString(p) + " r " + Arrays.toString(r) + "\n";
     }
-
-    // Tests
-    public static void main(String[] args) {
-        UnionFind uf = new UnionFind(5);
-        System.out.println("init /w 5 (should print 'p [0, 1, 2, 3, 4] r [0, 0, 0, 0, 0]'):");
-        System.out.println(uf);
-
-        uf.union(1, 2);
-        System.out.println("union 1 2 (should print 'p [0, 1, 1, 3, 4] r [0, 1, 0, 0, 0]'):");
-        System.out.println(uf);
-
-        uf.union(3, 4);
-        System.out.println("union 3 4 (should print 'p [0, 1, 1, 3, 3] r [0, 1, 0, 1, 0]'):");
-        System.out.println(uf);
-
-        uf.find(4);
-        System.out.println("find 4 (should print 'p [0, 1, 1, 3, 3] r [0, 1, 0, 1, 0]'):");
-        System.out.println(uf);
-
-        System.out.println("count (should print '3'):");
-        System.out.println(uf.count());
-    }
 }

src/test/java/com/thealgorithms/searches/UnionFindTest.java

@@ -0,0 +1,81 @@
+package com.thealgorithms.searches;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+class UnionFindTest {
+    private UnionFind uf;
+
+    @BeforeEach
+    void setUp() {
+        uf = new UnionFind(10); // Initialize with 10 elements
+    }
+
+    @Test
+    void testInitialState() {
+        // Verify that each element is its own parent and rank is 0
+        assertEquals("p [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] r [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]\n", uf.toString());
+        assertEquals(10, uf.count(), "Initial count of disjoint sets should be 10.");
+    }
+
+    @Test
+    void testUnionOperation() {
+        uf.union(0, 1);
+        uf.union(1, 2);
+        assertEquals(8, uf.count(), "Count should decrease after unions.");
+        assertEquals(0, uf.find(2), "Element 2 should point to root 0 after unions.");
+    }
+
+    @Test
+    void testUnionWithRank() {
+        uf.union(0, 1);
+        uf.union(1, 2); // Make 0 the root of 2
+        uf.union(3, 4);
+        uf.union(4, 5); // Make 3 the root of 5
+        uf.union(0, 3); // Union two trees
+
+        assertEquals(5, uf.count(), "Count should decrease after unions.");
+        assertEquals(0, uf.find(5), "Element 5 should point to root 0 after unions.");
+    }
+
+    @Test
+    void testFindOperation() {
+        uf.union(2, 3);
+        uf.union(4, 5);
+        uf.union(3, 5); // Connect 2-3 and 4-5
+
+        assertEquals(2, uf.find(3), "Find operation should return the root of the set.");
+        assertEquals(2, uf.find(5), "Find operation should return the root of the set.");
+    }
+
+    @Test
+    void testCountAfterMultipleUnions() {
+        uf.union(0, 1);
+        uf.union(2, 3);
+        uf.union(4, 5);
+        uf.union(1, 3); // Connect 0-1-2-3
+        uf.union(5, 6);
+
+        assertEquals(5, uf.count(), "Count should reflect the number of disjoint sets after multiple unions.");
+    }
+
+    @Test
+    void testNoUnion() {
+        assertEquals(10, uf.count(), "Count should remain 10 if no unions are made.");
+    }
+
+    @Test
+    void testUnionSameSet() {
+        uf.union(1, 2);
+        uf.union(1, 2); // Union same elements again
+
+        assertEquals(9, uf.count(), "Count should not decrease if union is called on the same set.");
+    }
+
+    @Test
+    void testFindOnSingleElement() {
+        assertEquals(7, uf.find(7), "Find on a single element should return itself.");
+    }
+}