class Solution {
public:
    /**
    * @param root: The root of tree
    * @return: the head of doubly list node
    */
    DoublyListNode* bstToDoublyList(TreeNode* root) {
        // Write your code here

        return getDouble(root);

    }

    DoublyListNode* getDouble(TreeNode* node){
        DoublyListNode* root=NULL;
        DoublyListNode* prenode = NULL;
        DoublyListNode* curnode = NULL;
        stack<TreeNode*> rightnodestack;
        TreeNode* tmp = node;
        while (tmp!=NULL){
            rightnodestack.push(tmp);
            tmp = tmp->left;
        }
        while (rightnodestack.empty()==false){
            if (root == NULL){
                root = new DoublyListNode(rightnodestack.top()->val);
                root->prev = NULL;
                prenode = root;
            }
            else{
                curnode = new DoublyListNode(rightnodestack.top()->val);
                curnode->prev = prenode;
                prenode->next = curnode;
                prenode = curnode;
            }
            //cout<<rightnodestack.top()->val<<endl;
            TreeNode* tmp = rightnodestack.top()->right;
            rightnodestack.pop();
            while (tmp!=NULL){
                rightnodestack.push(tmp);
                tmp = tmp->left;
            }
        }
        return root;
    }
};