Problem Flatten Binary Tree

 1 public void flatten(TreeNode root) {
 2         if (root == null) return;
 3         List<TreeNode> queue = new ArrayList<TreeNode>();
 4         Stack<TreeNode> stack = new Stack<TreeNode>();
 5         stack.push(null);
 6         TreeNode top  =root;
 7         while (top != null) {
 8             queue.add(top);
 9             if (top.right != null) stack.push(top.right);
10             if (top.left != null) stack.push(top.left);
11             top = stack.pop();
12         }
13 
14         for (int i = 0; i < queue.size(); i++) {
15             TreeNode node = queue.get(i);
16             node.left = null;
17             if (i < queue.size() - 1)  
18                 node.right = queue.get(i+1);
19         }
20     }