public class TreeNodeDemo {

    private static TreeNode head = null;
    private static TreeNode tail = null;

    public static void main(String[] args) {
        TreeNode tn10 = new TreeNode(10);
        TreeNode tn15 = new TreeNode(15);
        TreeNode tn5 = new TreeNode(5);
        TreeNode tn12 = new TreeNode(12);
        TreeNode tn20 = new TreeNode(20);
        TreeNode tn18 = new TreeNode(18);
        TreeNode tn25 = new TreeNode(25);

        tn15.left = tn10;
        tn15.right = tn20;
        tn10.left = tn5;
        tn10.right = tn12;
        tn20.left = tn18;
        tn20.right = tn25;
        TreeNode node = transTreeToList(tn15);
        while(node.right != null){
            System.out.print(node.val+" ");
            node = node.right;
        }

    }

    /**
     * root是二叉树的根，转换为双向链表要返回链表的头节点，不然找不到
     * @param root
     */
    public static TreeNode transTreeToList(TreeNode root){
        if(root == null)
            return null;
        transTreeToList(root.left);
        if(head == null){
            head = root;
            tail = root;
        }else{
            root.left = tail;
            tail.right = root;
            tail = root;
        }
        transTreeToList(root.right);
        return head;
    }


    private static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        public TreeNode(int val) {
            this.val = val;
        }
    }
}

 public static TreeNode convertWithStack(TreeNode root){
        if(root == null)
            return null;
        TreeNode cur = root;
        TreeNode pre = null;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while(!stack.isEmpty() || cur != null){
            while(cur != null){
                stack.push(cur);
                cur = cur.left;
            }
            cur = stack.pop();
            if(pre == null){
                root = cur;
                pre = cur;
            }else{
                pre.right = cur;
                cur.left = pre;
                pre = cur;
            }
            cur = cur.right;
        }
        return root;
    }