C 中的字符比较

Question

因此，我尝试执行此功能，只要用户输入唯一的单位字母，即可将单位添加到链接列表中。我的代码可以编译。但在第一次运行后，它只说“单位字母已经存在”，即使它应该还不存在。有人可以告诉我我哪里出错了吗？任何帮助，将不胜感激!

void addU(NODE** head){
    NODE *newNode, *temphead, *current;
    int check = 0;
    char ul, nl;
    newNode = (NODE*)malloc(sizeof(NODE));
    newNode->unit = malloc(sizeof(UNIT));

    /*Get unit details*/
    printf("\n\n\tUnit Letter: ");
    scanf(" %c", &(newNode->unit->letter));
    printf("\n\tMaximum number of occupants: ");
    scanf("%d", &(newNode->unit->max));

    /*Check if unit letter is unique*/
    temphead = current = *head;
    if (temphead!=NULL){
        while (current!=NULL){
            ul = tolower(current->unit->letter);    //convert to small case for comparison
            nl = tolower(newNode->unit->letter);
            if (ul == nl){
                check=1;
                printf("\n\n\tInvalid: Unit letter already exists.\n"); 
                break;
            }else current = current->next;
    }}

    /*Add newNode to list if unit letter is unique*/
    if (check==0){
        if (*head==NULL){
            newNode->next = NULL;
            *head = newNode;
        }else{
            newNode->next = *head;
            *head = newNode;
        }
        printf("\n\n\tUnit successfully added!\n");
    }
    free(newNode);
    free(newNode->unit);
}

Answer 1

如果在列表中插入新节点，请不要立即销毁它。