Hard

# 97. Construct Binary Search Tree from a given Preorder Traversal using Recursion

Objective: - Given a preorder traversal, construct BST from that.

Input: Preorder traversal

Similar Problem: This problem is similar to the - Construct Binary Search Tree from a given Preorder Traversal Using Stack (Without Recursion)

Approach:

The solution to the problem is similar to isBST Max-Min Solution.

"Your root value can have any value between -∞ to + ∞, say it is 30 here, When you val­i­date the right child of 30, it can take any value between 30 and + ∞. When you val­i­date the left child of 30, it can take any value between — ∞ and 30. likewise

So the idea is to Pass the min­i­mum and max­i­mum val­ues between which the node’s value must lie.

• Example: int[] preOrder = { 20, 10, 5, 1, 7, 15, 30, 25, 35, 32, 40 };
• The first element in the preorder[] will be the root, which is 20 here.
• we start with the range min­i­mum = Integer.MIN_VALUE and max­i­mum = Interger.MAX_VALUE, so your root can take any value between this range.
• So when putting the left node of 20(root), it must lie within the range of min­i­mum = Integer.MIN_VALUE and max­i­mum = 20, so check the next element in the preorder[], if it lies in this range, make it the left child to the root, else it must be the right child of the root, and so on. See the fig­ure for a bet­ter understanding. ( see the execution sequence)
• Solve it recursively.

Time Complexity: O(n)

Output:

```Inorder Traversal of Constructed Tree :
1 5 7 10 15 20 25 30 32 35 40```