Given preorder and inorder traversal results, construct the binary tree.
Sample Input:
[3,9,20,15,7], [9,3,15,20,7]
Sample Output:
[3,9,20,null,null,15,7]
Hints
Hint 1
First element in preorder is root
Hint 2
Find root in inorder to separate left and right subtrees
Hint 3
Recursively build left and right subtrees
