1008. Construct Binary Search Tree from Preorder Traversal

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

Return the root node of a binary search tree that matches the given preorder traversal.
(Recall that a binary search tree is a binary tree where for every node, any descendant of node.left has a value < node.val, and any descendant of node.right has a value > node.val.  Also recall that a preorder traversal displays the value of the node first, then traverses node.left, then traverses node.right.)

Example 1:
Input: [8,5,1,7,10,12]
Output: [8,5,10,1,7,null,12]

Note: 
  1. 1 <= preorder.length <= 100
  2. The values of preorder are distinct.
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Intuition
Build the tree recursively
Left subtree has nodes less than root
Right subtree has nodes less than upperBound

Terminate recursion when index > Array length OR Array element > upper Bound
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Time - O(N)
Space - O(Log N) - recursion stack trace
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