二叉搜索树与双向链表

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。

/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

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

    }

}
*/
public class Solution {
    public TreeNode Convert(TreeNode pRootOfTree) {
    TreeNode lastlist = covertNode(pRootOfTree,null);
    TreeNode pHead = lastlist;
    while (pHead != null && pHead.left != null) {
        pHead = pHead.left;
    }
    return pHead;
}

public TreeNode covertNode(TreeNode root, TreeNode lastlist) {
    if (root == null)
        return null;
    TreeNode cur = root;
    if (cur.left != null) {
        lastlist = covertNode(cur.left,lastlist);
    }
    cur.left = lastlist;
    if (lastlist != null) {
        lastlist.right = cur;
    }
    lastlist = cur;
    if (cur.right != null) {
        lastlist = covertNode(cur.right,lastlist);
    }
    return lastlist;
}
}