Add validate binary search tree solution

Author: hyunjung-choi

validate-binary-search-tree/hyunjung-choi.kt

@@ -0,0 +1,30 @@
+/**
+ * Example:
+ * var ti = TreeNode(5)
+ * var v = ti.`val`
+ * Definition for a binary tree node.
+ * class TreeNode(var `val`: Int) {
+ *     var left: TreeNode? = null
+ *     var right: TreeNode? = null
+ * }
+ */
+class Solution {
+    fun isValidBST(root: TreeNode?): Boolean {
+        val values = mutableListOf<Int>()
+        inorderTraversal(root, values)
+
+        for (i in 1 until values.size) {
+            if (values[i] <= values[i - 1]) return false
+        }
+
+        return true
+    }
+
+    private fun inorderTraversal(root: TreeNode?, values: MutableList<Int>) {
+        if (root == null) return
+
+        inorderTraversal(root.left, values)
+        values.add(root.`val`)
+        inorderTraversal(root.right, values)
+    }
+}