From cbdac306433dec51c99cc0b363dcfd8d063f78c3 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Sat, 21 Nov 2020 14:21:05 +0000
Subject: [PATCH] Reimplement bst-checker

Practice makes perfect. See the previous commit for a more details about this
solution.
---
 scratch/facebook/interview-cake/bst-checker.py | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
 create mode 100644 scratch/facebook/interview-cake/bst-checker.py

diff --git a/scratch/facebook/interview-cake/bst-checker.py b/scratch/facebook/interview-cake/bst-checker.py
new file mode 100644
index 000000000..bbd52fa9c
--- /dev/null
+++ b/scratch/facebook/interview-cake/bst-checker.py
@@ -0,0 +1,14 @@
+def is_valid(node):
+    """
+    Return True if `node` is a valid binary search tree.
+    """
+    s = []
+    s.append((float('-inf'), node, float('inf')))
+    while s:
+        lo, node, hi = s.pop()
+        if lo <= node.value <= hi:
+            node.lhs and s.append((lo, node.lhs, node.value))
+            node.rhs and s.append((node.value, node.rhs, hi))
+        else:
+            return False
+    return True