Enhance docs, add tests in AVLTree (#6058)

src/test/java/com/thealgorithms/datastructures/trees/AVLTreeTest.java

@@ -0,0 +1,101 @@
+package com.thealgorithms.datastructures.trees;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.List;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class AVLTreeTest {
+    private AVLTree avlTree;
+
+    @BeforeEach
+    public void setUp() {
+        avlTree = new AVLTree();
+    }
+
+    @Test
+    public void testInsert() {
+        assertTrue(avlTree.insert(10));
+        assertTrue(avlTree.insert(20));
+        assertTrue(avlTree.insert(5));
+        assertFalse(avlTree.insert(10)); // Duplicate
+    }
+
+    @Test
+    public void testSearch() {
+        avlTree.insert(15);
+        avlTree.insert(25);
+        assertTrue(avlTree.search(15));
+        assertFalse(avlTree.search(30)); // Not in the tree
+    }
+
+    @Test
+    public void testDeleteLeafNode() {
+        avlTree.insert(10);
+        avlTree.insert(20);
+        avlTree.insert(30);
+        avlTree.delete(30);
+        assertFalse(avlTree.search(30));
+    }
+
+    @Test
+    public void testDeleteNodeWithOneChild() {
+        avlTree.insert(20);
+        avlTree.insert(10);
+        avlTree.insert(30);
+        avlTree.delete(10);
+        assertFalse(avlTree.search(10));
+    }
+
+    @Test
+    public void testDeleteNodeWithTwoChildren() {
+        avlTree.insert(20);
+        avlTree.insert(10);
+        avlTree.insert(30);
+        avlTree.insert(25);
+        avlTree.delete(20);
+        assertFalse(avlTree.search(20));
+        assertTrue(avlTree.search(30));
+        assertTrue(avlTree.search(25));
+    }
+
+    @Test
+    public void testReturnBalance() {
+        avlTree.insert(10);
+        avlTree.insert(20);
+        avlTree.insert(5);
+        List<Integer> balances = avlTree.returnBalance();
+        assertEquals(3, balances.size()); // There should be 3 nodes
+        assertEquals(0, balances.get(0)); // Balance for node 5
+        assertEquals(0, balances.get(1)); // Balance for node 10
+        assertEquals(0, balances.get(2)); // Balance for node 20
+    }
+
+    @Test
+    public void testInsertAndRebalance() {
+        avlTree.insert(30);
+        avlTree.insert(20);
+        avlTree.insert(10); // This should cause a right rotation
+        assertTrue(avlTree.search(20));
+        assertTrue(avlTree.search(10));
+        assertTrue(avlTree.search(30));
+    }
+
+    @Test
+    public void testComplexInsertionAndDeletion() {
+        avlTree.insert(30);
+        avlTree.insert(20);
+        avlTree.insert(10);
+        avlTree.insert(25);
+        avlTree.insert(5);
+        avlTree.insert(15);
+
+        avlTree.delete(20); // Test deletion
+        assertFalse(avlTree.search(20));
+        assertTrue(avlTree.search(30));
+        assertTrue(avlTree.search(25));
+    }
+}