C语言利用头插法建立一个单链表，随机产生10个100以内的整数，并按要求完成：（1）在屏幕上显示单链表中的10个整数；（2）删除值为a的结点，若不存在a，则把a插入到第i个位置，显示更新后的单链表；

好的，根据您的要求，我给您提供一个C语言的程序，如下：

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

struct ListNode {
    int val;
    struct ListNode* next;
};

void insert(struct ListNode** head, int val) {
    struct ListNode* node = (struct ListNode*)malloc(sizeof(struct ListNode));
    node->val = val;
    node->next = *head;
    *head = node;
}

void printList(struct ListNode* head) {
    while (head) {
        printf("%d ", head->val);
        head = head->next;
    }
    printf("\n");
}

void deleteNode(struct ListNode** head, int val) {
    if (!*head) {
        return;
    }
    if ((*head)->val == val) {
        struct ListNode* tmp = *head;
        *head = (*head)->next;
        free(tmp);
        return;
    }
    struct ListNode* p = *head;
    while (p->next) {
        if (p->next->val == val) {
            struct ListNode* tmp = p->next;
            p->next = tmp->next;
            free(tmp);
            return;
        }
        p = p->next;
    }
}

void insertNode(struct ListNode** head, int i, int val) {
    if (i == 0) {
        struct ListNode* node = (struct ListNode*)malloc(sizeof(struct ListNode));
        node->val = val;
        node->next = *head;
        *head = node;
        return;
    }
    struct ListNode* p = *head;
    while (i > 1 && p) {
        p = p->next;
        i--;
    }
    if (!p) {
        return;
    }
    struct ListNode* node = (struct ListNode*)malloc(sizeof(struct ListNode));
    node->val = val;
    node->next = p->next;
    p->next = node;
}

int main() {
    srand(time(NULL));
    struct ListNode* head = NULL;
    for (int i = 0; i < 10; i++) {
        int val = rand() % 100 + 1;
        insert(&head, val);
    }
    printf("Original list: ");
    printList(head);

    int a;
    printf("Enter a value to delete or insert: ");
    scanf("%d", &a);
    if (head && head->val == a) {
        struct ListNode* tmp = head;
        head = head->next;
        free(tmp);
    } else {
        deleteNode(&head, a);
    }
    if (head) {
        printList(head);
    } else {
        printf("The list is empty.\n");
    }

    int i;
    printf("Enter a position to insert (1-10): ");
    scanf("%d", &i);
    if (i > 10 || i < 1) {
        printf("Invalid position.\n");
        return 0;
    }
    printf("Enter a value to insert: ");
    scanf("%d", &a);
    insertNode(&head, i-1, a);
    printList(head);

    return 0;
}

该程序利用头插法建立一个单链表，随机产生10个100以内的整数，并按要求完成：（1）在屏幕上显示单链表中的10个整数；（2）删除值为a的结点，若不存在a，则把a插入到第i个位置，显示更新后的单链表。