大佬能帮下忙吗？ 单链表出现的读取访问权限冲突

# include <stdio.h>
# include <stdlib.h>
# include <stdbool.h>

typedef struct LNode {
    int elem;
    struct LNode* next;
}Node, * List;

List init() {
    List list = (List)malloc(sizeof(Node));
    list->elem = 0;   list->next = NULL;
    return list;
}

int length(List list) {
    return list->elem;
}

bool isEmpty(List list) {
    return list->next == NULL;
}

void add(List list, int elem) {
    Node node = { elem,NULL };
    node.next = list->next;
    list->next = &node;
    list->elem++;
}

void append(List list, int elem) {
    Node node = { elem,NULL };
    List cur = (List)malloc(sizeof(Node));
    cur = list->next;
    while (cur != NULL) {
        cur = cur->next;
    }//结束while语句后，cur是一个nullptr
    cur = &node;
    list->elem++;
}


int* travel(List list) {
    int* num = (int*)malloc(sizeof(int) * length(list));//初始化num数组
    int i = 1;
    if (isEmpty(list)) {
        printf("链表为空！");
        num = NULL;
    }
    else {
        List cur = (List)malloc(sizeof(Node));
        if (!cur) {
            printf("申请内存失败！");
            exit(0);
        }
        cur = list->next;
        while (cur != NULL) {
            printf("链表第%d个节点为：%d\n", i, cur->elem);
            num[i - 1] = cur->elem;
            i++;
            cur = cur->next;
        }
    }
    return num;
}

int main() {
    List list = init();
    add(list, 3);
    add(list, 2);
    add(list, 1);
    append(list, 4);
    printf("链表是否为空：%d\n", isEmpty(list));
    printf("链表长度为:%d\n", length(list));
    travel(list);
    return 0;
}