add: 验证二叉搜索树

src/tree/validate-binary-search-tree.js

@@ -0,0 +1,16 @@
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {boolean}
+ */
+export const isValidBST = function (root, lower = -Infinity, upper = Infinity) {
+  if (root === null) return true
+  if (root.val <= lower || root.val >= upper) return false
+  return isValidBST(root.left, lower, root.val) && isValidBST(root.right, root.val, upper)
+}