fix: synchronize the Java solution

No.0230.Kth Smallest Element in a BST

Author: YangFong

solution/0200-0299/0230.Kth Smallest Element in a BST/README.md

@@ -118,6 +118,57 @@ class Solution {
 }
 ```
 
+```java
+/**
+ * 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 {
+    public int kthSmallest(TreeNode root, int k) {
+        int ans = -1;
+        while (root != null) {
+            if (root.left == null) {
+                --k;
+                if (k == 0) {
+                    ans = root.val;
+                    return ans;
+                }
+                root = root.right;
+            } else {
+                TreeNode pre = root.left;
+                while (pre.right != null && pre.right != root) {
+                    pre = pre.right;
+                }
+                if (pre.right == null) {
+                    pre.right = root;
+                    root = root.left;
+                } else {
+                    --k;
+                    if (k == 0) {
+                        ans = root.val;
+                        return ans;
+                    }
+                    pre.right = null;
+                    root = root.right;
+                }
+            }
+        }
+        return ans;
+    }
+}
+```
+
 ### **C++**
 
 ```cpp