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) {
                if(num==null||num.length==0){
            return null;
        }
        int length=num.length;
        TreeNode treenode=sortedArrayToBST(num,0,length-1);//我每次总是忘记减一，以后要记住
        return treenode ;

    }

    private TreeNode sortedArrayToBST(int[] num, int star, int tail) {
          if(star>tail){
            return null;
        }
        int mid=star+(tail-star+1)/2;
        TreeNode root=new TreeNode(num[mid]);
        root.left=sortedArrayToBST(num,star,mid-1);
        root.right=sortedArrayToBST(num,mid+1,tail);
        return root;
    }
}