Update 二叉树相关.md

blogs/Algorithm/剑指 Offer/二叉树相关.md

@@ -510,3 +510,47 @@ class Solution {
 }
 ```
 
+#### [Offer 54. 二叉搜索树的第k大节点](https://leetcode-cn.com/problems/er-cha-sou-suo-shu-de-di-kda-jie-dian-lcof/)
+
+```java
+class Solution {
+    List<Integer> list = new ArrayList<>();
+
+    public int kthLargest(TreeNode root, int k) {
+        helper(root);
+        return list.get(list.size() - k);
+    }
+
+    private void helper(TreeNode root) {
+        if (root == null) {
+            return;
+        }
+        helper(root.left);
+        list.add(root.val);
+        helper(root.right);
+    }
+}
+```
+
+```java
+class Solution {
+    int result = 0, count = 1;
+
+    public int kthLargest(TreeNode root, int k) {
+        helper(root, k);
+        return result;
+    }
+
+    private void helper(TreeNode root, int k) {
+        if (root == null) {
+            return;
+        }
+        helper(root.right, k);
+        if (count++ == k) {
+            result = root.val;
+            return;
+        }
+        helper(root.left, k);
+    }
+}
+```