原题链接: https://oj.leetcode.com/problems/convert-sorted-array-to-binary-search-tree/

public TreeNode sortedArrayToBST(int[] num) {
    if (num == null || num.length == 0) return null;

    List<Integer> cursor = new ArrayList<Integer>(1);
    cursor.add(0);

    return sortedArrayToBSTHelper(num, 0, num.length - 1, cursor);
}

public TreeNode sortedArrayToBSTHelper(int[] num, int low, int high, List<Integer> cursor) {
    if (low > high) return null;

    int mid = (low + high) / 2;

    TreeNode left = sortedArrayToBSTHelper(num, low, mid - 1, cursor);

    TreeNode root = new TreeNode(num[cursor.get(0)]);
    root.left = left;

    cursor.set(0, cursor.get(0) + 1);

    TreeNode right = sortedArrayToBSTHelper(num, mid + 1, high, cursor);
    root.right = right;

    return root;
}