class Solution {

public:

    void ConvertNode(TreeNode* pRootOfTree,TreeNode** pre)

    {

        if(pRootOfTree)

        {

            TreeNode* cur=pRootOfTree;

            if(cur->left)

                ConvertNode(cur->left,pre);

            cur->left=*pre;

            if(*pre)

                (*pre)->right=cur;

            *pre=cur;

            if(cur->right)

                ConvertNode(cur->right,pre);

        }

    }

    TreeNode* Convert(TreeNode* pRootOfTree)

    {

        if(!pRootOfTree)

            return NULL;

        TreeNode* pre=NULL;

        ConvertNode(pRootOfTree,&pre);

        while(pre->left)

        {

            pre=pre->left;

        }

        return pre;

    }

};