Update README.md

Author: Geekhyt

README.md

@@ -70,6 +70,7 @@
 - [102. 二叉树的层序遍历](https://github.com/Geekhyt/javascript-leetcode/issues/18)
 - [104. 二叉树的最大深度](https://github.com/Geekhyt/javascript-leetcode/issues/19)
 - [226. 翻转二叉树](https://github.com/Geekhyt/javascript-leetcode/issues/20)
+- [98. 验证二叉搜索树](https://github.com/Geekhyt/javascript-leetcode/issues/75)
 
 ## 栈