数据结构与算法 链式表的操作集

我的代码一直显示段错误，麻烦各位帮我看看是哪里出错了

Position Find( List L, ElementType X ){
    Position ptrl = L;
    while(ptrl != NULL && ptrl->Data != X){
        ptrl = ptrl->Next;
    }
    if( ptrl->Data == X){
        return ptrl;
    }
    else{
        return ERROR;
    }
}

List Insert( List L, ElementType X, Position P ){
    Position ptrl = L;
    List s = (List)malloc(sizeof(struct LNode));
    s->Data = X;
    if( P == L){
        s->Next = P;
        L = s;
        return L;
    }

    else if(  P == NULL){
        printf("Wrong Position for Insertion");
        return ERROR;
    }
    else{
        if( ptrl->Next!=P){
            ptrl = ptrl->Next;
        }
        else{
            ptrl->Next = s;
            s->Next = P;
            return L;
        }
    }
}


List Delete( List L, Position P ){
    Position ptrl = L;
    if( P == L){
        if( L != NULL){
            L = ptrl->Next;
        }
        else{
            return ERROR;
        }
        free( P);
        return L;
    }
    else if( P == NULL){
        return ERROR;
    }
    else if( ptrl != NULL && ptrl->Next != P){
        ptrl = ptrl->Next;
    }
    if( ptrl->Next == P){
        ptrl->Next = P->Next;
        free(P);
        return L;
    }
}

大体修改如下：

Position Find( List L, ElementType X ){
    Position ptrl = L;
    while(ptrl != NULL && ptrl->Data != X){
        ptrl = ptrl->Next;
    }
    if( ptrl->Data != NULL && ptrl->Data == X){ //修改1
        return ptrl;
    }
    else{
        return ERROR;
    }
}
List Insert( List L, ElementType X, Position P ){
    Position ptrl = L;
    List s = (List)malloc(sizeof(struct LNode));
    s->Data = X;
    if( P == L){
        s->Next = P;
        L = s;
        return L;
    }
    else if(  P == NULL){
        printf("Wrong Position for Insertion");
        return ERROR;
    }
    else{
        while( ptrl->Next!=P){ //修改2
            ptrl = ptrl->Next;
        }
         //修改3
            ptrl->Next = s;
            s->Next = P;
            return L;
        
    }
}

List Delete( List L, Position P ){
    Position ptrl = L;
    if( P == L){
        if( L != NULL){
            L = ptrl->Next;
        }
        else{
            return ERROR;
        }
        free( P);
        return L;
    }else if( P == NULL){
        return ERROR;
    }else
    {
        while(ptrl->Next != NULL && ptrl->Next != P)
            ptrl = ptrl->Next;
        if( ptrl->Next != NULL && ptrl->Next == P)
        {
            ptrl->Next = P->Next;
            free(P);
            return L;
        }
    }
    
}