diff --git a/Area.java b/Area.java
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/first-contributions b/first-contributions
new file mode 160000
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--- /dev/null
+++ b/first-contributions
@@ -0,0 +1 @@
+Subproject commit d58c69ca9ad0641a155f3a6c95efb484dc0a5abe
diff --git a/src/main/java/com/thealgorithms/datastructures/trees/InvertBinaryTree.java b/src/main/java/com/thealgorithms/datastructures/trees/InvertBinaryTree.java
new file mode 100644
index 000000000000..a72a3418fbd9
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+++ b/src/main/java/com/thealgorithms/datastructures/trees/InvertBinaryTree.java
@@ -0,0 +1,21 @@
+package com.thealgorithms.datastructures.trees;
+
+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 = root.right;
+        root.right = temp;
+
+        invertTree(root.left);
+        invertTree(root.right);
+        return root;
+    }
+}
diff --git a/src/main/java/com/thealgorithms/datastructures/trees/KthSmallestElementInBST.java b/src/main/java/com/thealgorithms/datastructures/trees/KthSmallestElementInBST.java
new file mode 100644
index 000000000000..d7aba6660be1
--- /dev/null
+++ b/src/main/java/com/thealgorithms/datastructures/trees/KthSmallestElementInBST.java
@@ -0,0 +1,41 @@
+package com.thealgorithms.datastructures.trees;
+
+/**
+ * Finds the kth smallest element in a Binary Search Tree.
+ *
+ * Time Complexity: O(n)
+ * Space Complexity: O(h)
+ */
+public final class KthSmallestElementInBST {
+
+    private KthSmallestElementInBST() {
+    }
+
+    public static int kthSmallest(BinaryTree.Node root, int k) {
+        Counter counter = new Counter();
+        BinaryTree.Node result = inorder(root, k, counter);
+        return result != null ? result.data : -1;
+    }
+
+    private static BinaryTree.Node inorder(BinaryTree.Node node, int k, Counter counter) {
+        if (node == null) {
+            return null;
+        }
+
+        BinaryTree.Node left = inorder(node.left, k, counter);
+        if (left != null) {
+            return left;
+        }
+
+        counter.count++;
+        if (counter.count == k) {
+            return node;
+        }
+
+        return inorder(node.right, k, counter);
+    }
+
+    private static class Counter {
+        int count = 0;
+    }
+}
diff --git a/src/test/java/com/thealgorithms/datastructures/trees/InvertBinaryTreeTest.java b/src/test/java/com/thealgorithms/datastructures/trees/InvertBinaryTreeTest.java
new file mode 100644
index 000000000000..255a819a0519
--- /dev/null
+++ b/src/test/java/com/thealgorithms/datastructures/trees/InvertBinaryTreeTest.java
@@ -0,0 +1,42 @@
+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;
+
+class InvertBinaryTreeTest {
+
+    @Test
+    void testInvertTreeNormalCase() {
+        BinaryTree.Node root = new BinaryTree.Node(4);
+        root.left = new BinaryTree.Node(2);
+        root.right = new BinaryTree.Node(7);
+        root.left.left = new BinaryTree.Node(1);
+        root.left.right = new BinaryTree.Node(3);
+
+        BinaryTree.Node inverted = InvertBinaryTree.invertTree(root);
+
+        assertEquals(7, inverted.left.data);
+        assertEquals(2, inverted.right.data);
+        assertEquals(3, inverted.right.left.data);
+        assertEquals(1, inverted.right.right.data);
+    }
+
+    @Test
+    void testInvertTreeSingleNode() {
+        BinaryTree.Node root = new BinaryTree.Node(1);
+
+        BinaryTree.Node inverted = InvertBinaryTree.invertTree(root);
+
+        assertEquals(1, inverted.data);
+        assertNull(inverted.left);
+        assertNull(inverted.right);
+    }
+
+    @Test
+    void testInvertTreeNull() {
+        BinaryTree.Node inverted = InvertBinaryTree.invertTree(null);
+
+        assertNull(inverted);
+    }
+}
diff --git a/src/test/java/com/thealgorithms/datastructures/trees/KthSmallestElementBSTTest.java b/src/test/java/com/thealgorithms/datastructures/trees/KthSmallestElementBSTTest.java
new file mode 100644
index 000000000000..8ca9d07f737d
--- /dev/null
+++ b/src/test/java/com/thealgorithms/datastructures/trees/KthSmallestElementBSTTest.java
@@ -0,0 +1,17 @@
+package com.thealgorithms.datastructures.trees;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class KthSmallestElementBSTTest {
+
+    @Test
+    void simpleTest() {
+        BinaryTree.Node root = new BinaryTree.Node(5);
+        root.left = new BinaryTree.Node(3);
+        root.right = new BinaryTree.Node(7);
+
+        assertEquals(3, KthSmallestElementInBST.kthSmallest(root, 1));
+    }
+}