From 15e6d58b120665f05fa12746914e6d5ec25b50f7 Mon Sep 17 00:00:00 2001
From: Matej Focko <mfocko@redhat.com>
Date: Thu, 11 Aug 2022 10:02:47 +0200
Subject: [PATCH] =?UTF-8?q?problems:=20add=20=E2=80=9E98.=20Validate=20Bin?=
 =?UTF-8?q?ary=20Search=20Tree=E2=80=9C?=
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Signed-off-by: Matej Focko <mfocko@redhat.com>
---
 problems/validate-binary-search-tree.kt | 26 +++++++++++++++++++++++++
 1 file changed, 26 insertions(+)
 create mode 100644 problems/validate-binary-search-tree.kt

diff --git a/problems/validate-binary-search-tree.kt b/problems/validate-binary-search-tree.kt
new file mode 100644
index 0000000..9b9058d
--- /dev/null
+++ b/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())
+}