管理链接列表的函数

我有一个名为StringList的类，它有多个函数来管理和操作字符串的链表。我有一个AddtoTop，它添加到列表的顶部，AddtoBottom，一个名为add的函数，它添加一个字符串，但按字母顺序排列，一个clear函数，一个find函数，print和一个remove函数。我写过每一个函数，但我有一个主要问题。当我使用"AddtoTop"answers"AddtoBottom"时，我会在链表中添加一个字符串。当我要求删除我只使用这两个函数放置的字符串时，我的remove函数就起作用了。当我试图删除我用"add"函数添加的单词时，我的程序崩溃了。函数"remove"（如下所示）可以删除使用"AddtoBottom"answers"AddtoTop"函数添加的字符串，但不能删除使用"add"添加的字符串。有人能帮我找出原因吗？我试着用不同的方式重新写，但我被卡住了。以下是我的功能：

StringList::StringList() //constructor
{
    pTop=NULL;
    pBottom=NULL;
}
StringList::~StringList()  //destructor
{
    StringListNode *next;
    for (StringListNode *sp = pTop; sp != 0; sp = next)
    {
        next = sp->pNext;
        delete sp;
    }
}
void StringList::add(string s) //adds and places in alphabetical order
{
    if(pTop)
    {
      if( s < pTop->data )
        {
            StringListNode *A=new StringListNode;
            A->data = s;
            A->pNext = pTop;
            pTop = A;   // new top
            return;
        }
        // go further into list
        StringListNode *iter = pTop;
        while( iter->pNext )    
        {
            if(  iter->pNext->data < s )
                iter = iter->pNext;
            else
                break;
        }   
        StringListNode *in=new StringListNode;   //actuallly inserts node
        in->data = s;
        in->pNext = iter->pNext;
        iter->pNext = in;
    }
    else// new item 
    {
        pTop = new StringListNode;
        pTop->data = s;
        pTop->pNext = NULL;
    }
}
StringList::StringListNode *StringList::find(const string &s) //basic search function
{
    StringListNode *sp = pTop;   // Search
    while (sp != 0 && sp->data != s)
        sp = sp->pNext;
    return sp;
}
void StringList::addToTop(string s) //add to top of nodes
{
    if(isEmpty())
    {
        StringListNode * pNewNode;
        pNewNode = new StringListNode;
        (*pNewNode).data = s;
        pTop=pNewNode;
        pBottom=pNewNode;
        (*pNewNode).pPrev = NULL;
        (*pNewNode).pNext = NULL;
    }
    else //it's not empty
    {
        StringListNode * pNewNode;
        pNewNode = new StringListNode;
        (*pNewNode).data = s;
        (*pNewNode).pNext = pTop;
        (*pTop).pPrev = pNewNode;
        (*pNewNode).pPrev =NULL;
        pTop=pNewNode;
    }
}
void StringList::addToBottom(string s) // add to bottom
{
    if(isEmpty())
    {
        StringListNode * pNewNode;
        pNewNode = new StringListNode;
        (*pNewNode).data = s;
        pTop=pNewNode;
        pBottom=pNewNode;
        (*pNewNode).pPrev = NULL;
        (*pNewNode).pNext = NULL;
    }
    else
    {
        StringListNode * pNewNode;
        pNewNode = new StringListNode;
        (*pNewNode).data = s;
        (*pBottom).pNext = pNewNode;
        (*pNewNode).pPrev =  pBottom;
        (*pNewNode).pNext =NULL;
        pBottom=pNewNode;
    }
}
string StringList::print() //prints strings in linked list
{
    string result;
    StringListNode * pCurrent;
    pCurrent=pTop;
    while(pCurrent!=NULL)
    {
        result+=(*pCurrent).data+"n";
        pCurrent=(*pCurrent).pNext;
    }
    return result;
}
void StringList::clear()   //clears everything
{
    pTop = NULL;
    pBottom = NULL;
}
void StringList::remove(string s)  //removes a string
{
    StringListNode *curr = this->find(s);
    if (curr->pPrev != 0)
        curr->pPrev->pNext = curr->pNext;
    if (curr->pNext != 0)
        curr->pNext->pPrev = curr->pPrev;
    if (pTop == curr)
        pTop = curr->pNext;
    if (pBottom == curr)
        pBottom = curr->pPrev;
}