Feat: 98. Validate Binary Search Tree

Author: HC-kang

validate-binary-search-tree/HC-kang.ts

@@ -0,0 +1,26 @@
+// class TreeNode {
+//   val: number;
+//   left: TreeNode | null;
+//   right: TreeNode | null;
+//   constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+//     this.val = val === undefined ? 0 : val;
+//     this.left = left === undefined ? null : left;
+//     this.right = right === undefined ? null : right;
+//   }
+// }
+
+/**
+ * https://leetcode.com/problems/validate-binary-search-tree
+ * T.C. O(n)
+ * S.C. O(n)
+ */
+function isValidBST(root: TreeNode | null): boolean {
+  return validate(root, null, null);
+}
+
+function validate(node: TreeNode | null, min: number | null, max: number | null): boolean {
+  if (!node) return true;
+  if (min !== null && node.val <= min) return false;
+  if (max !== null && node.val >= max) return false;
+  return validate(node.left, min, node.val) && validate(node.right, node.val, max);
+}