程序代码：

3.#include<stdio.h>
void palin(int n){
    char next;
    if(n<=1){
        next=getchar();
        printf("\n\0:");
        putchar(next);
    }
    else{
        next=getchar();
        palin(n-1);
        putchar(next);
    }
}
int main(){
    int i=5;
    palin(i);
    printf("\n");
    return 0;
}

程序代码：

#include <stdio.h>
#include <stdlib.h>
#define SIZE (sizeof(NODE))

typedef struct node {
    char ch;
    struct node *next;
} NODE, *PN;

void prtList(PN head) {
    PN p = NULL;

    if(head != NULL) {
        for(p = head; p != NULL; p = p->next)
            putchar(p->ch);

        puts("");

    } else puts("链表为空");
}

PN addToListHead(PN head, char ch) {
    PN pnew = (PN)malloc(SIZE);

    if(pnew == NULL) {
        puts("错误: 头部插入节点时开辟内存失败...");
        exit(EXIT_FAILURE);
    }

    pnew->ch = ch;
    pnew->next = head;
    return pnew;
}

PN createList(void) {
    PN head = NULL;
    char ch;

    while((ch = getchar()) != '\n')
        head = addToListHead(head, ch);

    return head;
}

void freeList(PN head) {
    PN p;

    while(head != NULL) {
        p = head->next;
        free(head);
        head = p;
    }
}

int main(void) {
    PN head = createList();
    prtList(head);
    freeList(head);
    return 0;
}