add: KthSmallestNumberInBSTRecursive

Author: Jer3myYu

java/Trees/KthSmallestNumberInBSTRecursive.java

@@ -0,0 +1,34 @@
+import java.util.ArrayList;
+import java.util.List;
+
+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class KthSmallestNumberInBSTRecursive {
+    public int kthSmallestNumberInBSTRecursive(TreeNode root, int k) {
+        List<Integer> sortedList = new ArrayList<>();
+        inorder(root, sortedList);
+        return sortedList.get(k - 1);
+    }
+
+    // Inorder traversal function to attain a sorted list of nodes from the BST.
+    private void inorder(TreeNode node, List<Integer> sortedList) {
+        if (node == null) {
+            return;
+        }
+        inorder(node.left, sortedList);
+        sortedList.add(node.val);
+        inorder(node.right, sortedList);
+    }
+}