解题思路：

class Solution {
    List<TreeNode> list = new ArrayList<>();

    public void flatten(TreeNode root) {
        recur(root);
        for (int i = 1; i < list.size(); i++) {
            TreeNode pre = list.get(i - 1);
            TreeNode cur = list.get(i);
            pre.left = null;
            pre.right = cur;
        }
    }

    public void recur(TreeNode root) {
        if (root == null) return;
        list.add(root);
        recur(root.left);
        recur(root.right);
    }
}