98. Validate Binary Search Tree (#76)

Author: Kohei Asai

solutions/validateBinarySearchTree.test.ts

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+import { createBinaryTreeNode } from "../utilities/TreeNode";
+import isValidBST from "./validateBinarySearchTree";
+
+describe("98. Validate Binary Search Tree", () => {
+  test("#1", () => {
+    expect(isValidBST(createBinaryTreeNode([2, 1, 3]))).toBe(true);
+  });
+
+  test("#2", () => {
+    expect(isValidBST(createBinaryTreeNode([5, 1, 4, null, null, 3, 6]))).toBe(
+      false
+    );
+  });
+
+  test("#3", () => {
+    expect(isValidBST(createBinaryTreeNode([1, 1]))).toBe(false);
+  });
+
+  test("#4", () => {
+    expect(isValidBST(createBinaryTreeNode([]))).toBe(true);
+  });
+});

solutions/validateBinarySearchTree.ts

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+import { TreeNode } from "../utilities/TreeNode";
+
+// 98. Validate Binary Search Tree
+// https://leetcode.com/problems/validate-binary-search-tree/
+export default function isValidBST(
+  node: TreeNode<number> | null,
+  greaterThan = -Infinity,
+  lessThan = Infinity
+): boolean {
+  if (node === null) return true;
+
+  return (
+    node.val > greaterThan &&
+    node.val < lessThan &&
+    isValidBST(node.left, greaterThan, node.val) &&
+    isValidBST(node.right, node.val, lessThan)
+  );
+}