convert sorted array/list to a height balanced BST.

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public TreeNode sortedArrayToBST(int[] num) {
// Start typing your Java solution below
// DO NOT write main() function
if(num==null)
return null;

int start = 0;
int end = num.length-1;

return sortedArrayToBST(num,start,end);
}

public TreeNode sortedArrayToBST(int[] num,int start,int end)
{
if(num==null)
return null;

while(start<=end)
{
int mid = start + (end-start)/2;
TreeNode n = new TreeNode(num[mid]);
n.left = sortedArrayToBST(num,start,mid-1);
n.right = sortedArrayToBST(num,mid+1,end);
return n;
}
return null;
}
}