108. Convert Sorted Array to Bin

思路：递归，二分查找
要把一个数组变为二分查找树，数组中间点就是根节点，依次递归构造左右子树。

 public static TreeNode sortedArrayToBST(int[] nums) {
        if (nums.length == 0) {
            return null;
        }
        TreeNode head = helper(nums, 0, nums.length - 1);
        return head;
    }

    public static TreeNode helper(int[] nums, int low, int high) {
        if (low > high) {
            return null;
        }

        int mid = (low + high) / 2;
        TreeNode root = new TreeNode(nums[mid]);
        root.left = helper(nums, low, mid -1);
        root.right = helper(nums, mid + 1, high);
        return root;
    }