Given an array where elements are sorted in ascending order, convert it to a height balanced BST.
解題思路： dfs ，按照bst的定義，從nums的中點選做head， 然后左右兩邊去dfs

 public TreeNode sortedArrayToBST(int[] nums) {
      if (nums.length == 0) {
          return null;
      }
      TreeNode head = helper(nums, 0, nums.length - 1);
      return head;
    }
    public TreeNode helper(int[] nums, int low, int high) {
        if (low > high) { // Done
            return null;
        }
        int mid = (low + high) /2;
        TreeNode node = new TreeNode(nums[mid]);
        node.left = helper(nums, low, mid - 1);
        node.right = helper(nums, mid + 1, high);
        return node;
    }