https://leetcode.com/problems/validate-binary-search-tree/

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

The left subtree of a node contains only nodes with keys less than the node's key. The right subtree of a node contains only nodes with keys greater than the node's key. Both the left and right subtrees must also be binary search trees. Example 1:

2

/ \

1 3

Input: [2,1,3] Output: true Example 2:

5

/ \

1 4

 / \

3   6

Input: [5,1,4,null,null,3,6] Output: false Explanation: The root node's value is 5 but its right child's value is 4.

https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/

Given preorder and inorder traversal of a tree, construct the binary tree.

Note: You may assume that duplicates do not exist in the tree.

Can you do it both iteratively and recursively?

For example, given

preorder = [3,9,20,15,7]

inorder = [9,3,15,20,7] Return the following binary tree:

3

/ \

9 20

/  \

15 7