feat: add InvertBinaryTree algorithm with tests

src/main/java/com/thealgorithms/datastructures/trees/InvertBinaryTree.java

@@ -0,0 +1,32 @@
+package com.thealgorithms.datastructures.trees;
+
+/**
+ * Inverts a binary tree by swapping the left and right children of every node.
+ *
+ * <p>Example:
+ * <pre>
+ *      4                4
+ *     / \              / \
+ *    2   7    ->      7   2
+ *   / \ / \          / \ / \
+ *  1  3 6  9        9  6 3  1
+ * </pre>
+ *
+ * <p>Time complexity: O(n), Space complexity: O(h) where h is tree height.
+ *
+ * @see <a href="https://leetcode.com/problems/invert-binary-tree/">LeetCode #226</a>
+ */
+public final class InvertBinaryTree {
+    private InvertBinaryTree() {
+    }
+
+    public static BinaryTree.Node invertTree(BinaryTree.Node root) {
+        if (root == null) {
+            return null;
+        }
+        BinaryTree.Node temp = root.left;
+        root.left = invertTree(root.right);
+        root.right = invertTree(temp);
+        return root;
+    }
+}

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

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+package com.thealgorithms.datastructures.trees;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import org.junit.jupiter.api.Test;
+
+public class InvertBinaryTreeTest {
+
+    @Test
+    public void testNullRoot() {
+        assertNull(InvertBinaryTree.invertTree(null));
+    }
+
+    @Test
+    public void testSingleNode() {
+        BinaryTree.Node root = TreeTestUtils.createTree(new Integer[] {1});
+        InvertBinaryTree.invertTree(root);
+        assertEquals(1, root.data);
+        assertNull(root.left);
+        assertNull(root.right);
+    }
+
+    /*
+         4                4
+        / \              / \
+       2   7    ->      7   2
+      / \ / \          / \ / \
+     1  3 6  9        9  6 3  1
+    */
+    @Test
+    public void testInvertTree() {
+        BinaryTree.Node root = TreeTestUtils.createTree(new Integer[] {4, 2, 7, 1, 3, 6, 9});
+        InvertBinaryTree.invertTree(root);
+        assertEquals(4, root.data);
+        assertEquals(7, root.left.data);
+        assertEquals(2, root.right.data);
+        assertEquals(9, root.left.left.data);
+        assertEquals(6, root.left.right.data);
+        assertEquals(3, root.right.left.data);
+        assertEquals(1, root.right.right.data);
+    }
+}