链表test

今天学了下链表，自己敲了一遍，有点小瑕疵，有点不明白为什么我的下标为0处不可以插入节点呢，是因为不能代替头结点吗，希望各位大牛们把我看一看

纯小白一只\(^o^)/，万分感谢

//程序内部存放一个头指针，一个地址，内存内部存放多段地址，第二个与头地址相连
#include <iostream>
#include <cstdio>
#include <time.h>

using namespace std;

#define ERROR  0
#define OK  1

typedef struct node{
    int data;
    struct node *next;
}linkedlist,*plist; //plist定义的就是指针型，所以后面的不用定义为*p之类的 

plist creat_node(int n) {//返回构造的地址，所以是plist类型 
    plist p = (plist)malloc(sizeof(linkedlist));
    p->data = n;
    p->next = NULL;
    return p;
}

plist clear_node( plist head ) {
    plist p = head;
    plist q;
    while ( p ) {
        q = p;
        free(p);
        p = q->next;
    }
} 
// 在head前面增加虚拟节点就可以少了一步特判index = 0的时候
plist insert_node (plist head, plist new_node, int index) {
    linkedlist ret;//建立虚拟节点，防止index = 0时没有办法向前走 
    ret.next = head;
    plist p = &ret;
    while (index > 0 && p != NULL) {
        p = p->next;
        index--;
    }
    if (p == NULL) {
        return ret.next;
    }
    new_node->next = p->next;
    p->next = new_node;
    return ret.next;
} 
// 因为需要前一个节点的信息,所以设置一个虚拟节点
plist delete_node (plist head, int x) {
    linkedlist ret;
    ret.next = head;
    plist p = &ret;
    plist q = head;
    while ( q && q->data != x) {//p一直在q前一位 
        p = p->next;
        q = q->next;
    }
    if ( q ) {//找到了，释放q 
        p->next = q->next;
        free(q);
        ret.data = OK;
    }
    return ret.next;
}

void output(plist head) {
    plist p = head;
    if( p == head ) {
        cout << head->data;
        p = p->next;
    }
    while ( p ) {
        cout << "->" <<p->data;
        p = p->next;
    }
    cout << endl;
}

int length_node(plist head) {
    plist p = head;
    int length;
    while ( p ) {
        p = p->next;
        length++;
    }
    return length;
}

int main( ) {
    int opr,n;
    srand(time(0));
    plist p;
    plist head = creat_node(rand() % 100 + 1);
    while (cin >> opr){
        switch (opr){
            case 0 :{
                        while (cin >> n ) {
                            if (n > 100)
                                break;
                            insert_node(head,creat_node(rand() % 100 + 1), n);
                            cout << length_node(head) << endl;
                            output(head);
                        }
    
                            break;
                    }
            case 1 :{
                            while (cin >> n) {
                                delete_node(head, n);
                                length_node(head);
                                output(head);    
                            }
                            
                            break;
                    }
            default :{
                        cout<<"ERROR";
                        break;
                    }
        }
    }
    clear_node(head);
    return 0;
}