add kth smallest element in a bst solution

Author: Tessa1217

kth-smallest-element-in-a-bst/Tessa1217.java

@@ -0,0 +1,51 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+
+    private int count = 0;
+
+    private int kthSmallValue = 0;
+
+    // 시간복잡도: O(k) (불균형 상태의 이진 트리일 경우 O(n))
+    public int kthSmallest(TreeNode root, int k) {
+        orderSearch(root, k);
+        return kthSmallValue;
+    }
+
+    // In Order Search
+    private void orderSearch(TreeNode node, int k) {
+
+        if (node == null) {
+            return;
+        }
+
+        // HINT => utilize the property of a BST => 좌측 리프 노드부터 탐색
+        orderSearch(node.left, k);
+
+        count++;
+
+        if (count == k) {
+            kthSmallValue = node.val;
+            return;
+        }
+
+        // search right side
+        orderSearch(node.right, k);
+
+    }
+
+}
+