Given a sorted (increasing order) array with unique integer elements, write an algo­rithm to create a binary search tree with minimal height.

Example:

Given sorted array: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5]，which represents the following tree: 

          0 
         / \ 
       -3   9 
       /   / 
     -10  5