114. Flatten Binary Tree to Linked List

class Solution {
    public void flatten(TreeNode root) {
        if (root == null) return ;
        TreeNode left = root.left;
        TreeNode right = root.right;
        root.left = null;
        flatten(left);
        flatten(right);
        root.right = left;
        TreeNode cur = root;
        while (cur.right != null ) cur = cur.right;
        cur.right = right;
    }
}