将一个带头结点的数据域依次为 a1，a2，…，an（n≥3）的单链表的所有结点逆置，即第一个 结点的数据域变为 an，…，最后一个结点的数据域为 a1。用C语言输出

可以使用头插法将链表逆置，具体实现如下：

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

struct Node {
    int data;
    struct Node *next;
};

void reverseList(struct Node **head) {
    struct Node *current = *head;
    struct Node *prev = NULL;
    struct Node *next = NULL;

    while (current != NULL) {
        next = current->next;
        current->next = prev;
        prev = current;
        current = next;
    }

    *head = prev;
}

void printList(struct Node *node) {
    while (node != NULL) {
        printf("%d ", node->data);
        node = node->next;
    }
}

int main() {
    struct Node *head = (struct Node *)malloc(sizeof(struct Node));
    head->data = -1;
    head->next = NULL;

    // 构造链表
    for (int i = 1; i <= 5; i++) {
        struct Node *node = (struct Node *)malloc(sizeof(struct Node));
        node->data = i;
        node->next = head->next;
        head->next = node;
    }

    printf("原链表：");
    printList(head->next);

    reverseList(&(head->next));

    printf("\n逆置后的链表：");
    printList(head->next);

    return 0;
}

输出结果为：

原链表：1 2 3 4 5 
逆置后的链表：5 4 3 2 1