原题链接: https://oj.leetcode.com/problems/flatten-binary-tree-to-linked-list/

public void flatten(TreeNode root) {
    if (root == null) return;

    List<TreeNode> pre = new ArrayList<TreeNode>(1);
    pre.add(null);
    flattenHelper(root, pre);
}

private void flattenHelper(TreeNode root, List<TreeNode> pre) {
    if (root == null) return;

    if (pre.get(0) != null) {
        pre.get(0).left = null;
        pre.get(0).right = root;
    }

    TreeNode rootRight = root.right;
    pre.set(0, root);
    flattenHelper(root.left, pre);
    flattenHelper(rootRight, pre);
}