如果现在要找出所有无效用户，那么针对上述的数据结构，写出代码

class UserInfo {
public:
    struct UserInfoLinkNode {
        UserInfoLinkNode(UserInfo * ui){
            info = ui;
            next = NULL;
        }
        UserInfo * info;
        struct UserInfoLinkNode * next;
    };
    UserInfo(int id){
        FriendListHead = NULL;
        FriendListTail = NULL;
        FriendCount = 0;
        FriendCount1 = 0;
        Id = id;
    }
    
    void AddFriend(UserInfo * ui) {
        if ( NULL==ui ) {
            return ;
        }
        if ( NULL==FriendListHead ) {
            FriendListHead = FriendListTail = new UserInfoLinkNode(ui);
        }
        else {
            FriendListTail->next = new UserInfoLinkNode(ui);
            FriendListTail = FriendListTail->next;
        }
        ++FriendCount;
        ui->FriendCount1++;
    }
    
    void DelFriend(UserInfo * ui) {
        if ( NULL==ui || NULL==FriendListHead ) {
            return ;
        }
        struct UserInfoLinkNode * p = FriendListHead;
        struct UserInfoLinkNode * p1 = p;
        while( NULL!=p ) {
            if ( ui==p->info ) {
                ui->FriendCount1--;
                --FriendCount;
                if ( p1==p ) {
                    if ( FriendListHead==FriendListTail ) {
                        FriendListHead = FriendListTail = NULL;
                    }
                    else {
                        FriendListHead = FriendListHead->next;
                    }
                }
                else {
                    if ( p==FriendListTail ) {
                        FriendListTail = p1;
                        FriendListTail->next = NULL;
                    }
                    else {
                        p1->next = p->next;
                    }
                }
                delete p;
                break;
            }
            p1 = p;
            p = p->next;
        }
    }
    
    bool IsFriend(int id) {
        struct UserInfoLinkNode * p = FriendListHead;
        while( NULL!=p ) {
            if ( id==p->info->Id ) {
                return true;
            }
            p = p->next;
        }
        return false;
    }
    
    int Id;
    struct UserInfoLinkNode * FriendListHead;
    struct UserInfoLinkNode * FriendListTail;
    int FriendCount;
    int FriendCount1;
};