Add KthSmallestElementInBST implementation with test

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;
+    }
+}

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));
+    }
+}