Construct Tree Solution

Implement tree construction from preorder and inorder arrays.

Optimization tips:

• Build a hash map: value → index in inorder
• Pass indices (preStart, preEnd, inStart, inEnd) instead of slicing
• This reduces time from $O(n^2)$ to $O(n)$

Here's the trick: the number of elements in the left subtree in inorder equals the number of elements after the root in preorder's left portion.

Test on trees of various shapes: complete, skewed left, skewed right, single node. Time: $O(n)$. Space: $O(n)$.