problems: add „98. Validate Binary Search Tree“

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/validate-binary-search-tree.kt

@@ -0,0 +1,26 @@
+/**
+ * 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 {
+    private data class SpecialRange(val min: Int? = null, val max: Int? = null) {
+        fun check(x: Int): Boolean =
+            (min == null || x > min) && (max == null || x < max)
+    }
+
+    private fun isValidBST(root: TreeNode?, range: SpecialRange): Boolean =
+        when (root) {
+            null -> true
+            else -> range.check(root.`val`)
+                && isValidBST(root.left, range.copy(max=root.`val`))
+                && isValidBST(root.right, range.copy(min=root.`val`))
+        }
+
+    fun isValidBST(root: TreeNode?): Boolean = isValidBST(root, SpecialRange())
+}