输入一棵二叉搜索树，将该二叉搜索树转换成一个排序的双向链表。要求不能创建任何新的结点，只能调整树中结点指针的指向。

输入一棵二叉搜索树，将该二叉搜索树转换成一个排序的双向链表。要求不能创建任何新的结点，只能调整树中结点指针的指向。

思想：利用非递归中序遍历思想

public TreeNode Convert(TreeNode pRootOfTree) {
        if (pRootOfTree == null) {
            return null;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode root = pRootOfTree;
        TreeNode head = null;
        TreeNode pre = null;
        boolean isFirst = true;
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if (isFirst) {
                head = root;
                pre = head;
                isFirst = false;
            } else {
                pre.right = root;
                root.left = pre;
                pre = root;
            }
            root = root.right;
        }
        return head;
    }