Add week 15 solutions: validate-binary-search-tree

Author: gitsunmin

validate-binary-search-tree/gitsunmin.ts

@@ -0,0 +1,32 @@
+/**
+ * https://leetcode.com/problems/validate-binary-search-tree/
+ * time complexity : O(n)
+ * space complexity : O(n)
+ */
+
+class TreeNode {
+    val: number;
+    left: TreeNode | null;
+    right: TreeNode | null;
+    constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+        this.val = val ?? 0;
+        this.left = left ?? null;
+        this.right = right ?? null;
+    }
+}
+
+function validate(node: TreeNode | null, lower: number, upper: number): boolean {
+    if (!node) return true;
+    const val = node.val;
+
+    if (val <= lower || val >= upper) return false;
+    if (!validate(node.right, val, upper)) return false;
+    if (!validate(node.left, lower, val)) return false;
+
+    return true;
+}
+
+function isValidBST(root: TreeNode | null): boolean {
+
+    return validate(root, -Infinity, Infinity);
+}