Problem Flatten Binary Tree

Problem Description: Given a binary tree, flatten it to a linked list in-place.

Solution: 对二叉树进行前序遍历(pre-order).

public void flatten(TreeNode root) {
        if (root == null) return;
        List<TreeNode> queue = new ArrayList<TreeNode>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(null);
        TreeNode top  =root;
        while (top != null) {
            queue.add(top);
            if (top.right != null) stack.push(top.right);
            if (top.left != null) stack.push(top.left);
            top = stack.pop();
        }

        for (int i = 0; i < queue.size(); i++) {
            TreeNode node = queue.get(i);
            node.left = null;
            if (i < queue.size() - 1)  
                node.right = queue.get(i+1);
        }
    }