添加/删除链表中的第一个元素

Question

我正在尝试插入/删除链表中的第一个元素，但在插入时它不会添加它，在删除时它会启动无限链，我无法识别问题。 我目前正在学习 DSA，所以请忽略代码中添加的任何不必要的注释

我创建的代码，（C语言）

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

struct Linked_List                      //structure has to be defined first
{
    int number;
    struct Linked_List *next;
};

typedef struct Linked_List node;

void create(node *p);
int count(node *p);
void print(node *p);
node *insert(node *head);
node *find(node *p, int key);
node *delete(node *head);

int main() {
    node *head;
    head = (node *)malloc(sizeof(node));
    create(head);
    printf("\n");
    print(head);
    printf("\n\nThe total number of elements in the linked list are :- %d",count(head));
    printf("\n");
    insert(head);
    printf("\n");
    print(head);
    printf("\n");
    delete(head);
    printf("\n");
    print(head);
}

void create(node *p) {
    printf("\nInput number (Input -999 to end) :- ");
    scanf("%d", &p->number);

    if (p->number == -999) {
        p->next = NULL;
    } else {
        p->next = (node *)malloc(sizeof(node));
        create(p->next);
    }
}

int count(node *p) {
    if (p->next == NULL) {            //using the if we are not counting the last element (-999)
        return 0;                   //return 1    will add the last element to total count
    } else {
        return 1 + count(p->next);
    }

    //return 1 + count(p->next);            //FAIL (as at last returns null pointer pointing to nowhere) returns total elements including -999,
}

void print(node *p) {
    
    // printf("%d --> ", p->number);        //this fails as in the end it will pass a null pointer which doesn't points to anywhere
    // print(p->next);
    
    if (p->next != NULL) {                        //this works as it first checks if pointer is null or not 
        printf("%d --> ", p->number);            //if not it prints the number and recursion occurs
        if (p->next->next == NULL) {              //this if statement is used to print the last value which holds null pointer as outer if will not process it,
            printf("%d", p->next->number);       //it checks for next value if null pointer then prints its value
        }
        print(p->next);
    }    
}

node *insert(node *head) {
    node *new;                                  //structure of list
    node *preceding;                            // preceding -> new -> key
    int x, key;

    printf("\nEnter the new value to be added :- ");
    scanf("%d", &x);
    printf("\nValue of key item (-999 if last) {-> new -> key}:- ");
    scanf("%d", &key);

    if (head->number == key) {                            //case for first position
        new = (node *)malloc(sizeof(node));
        new->number = x;
        new->next = head;
        head = new;
    } else {
        preceding = find(head, key);
        if (preceding == NULL) {
            printf("\nKey not found!!\n");
        } else {
            new = (node *)malloc(sizeof(node));
            new->number = x;
            new->next = preceding->next;
            preceding->next = new;
        }
    }
    return head;
}

node *delete(node *head) {
    int key;        //item to be deleted
    node *preceding;
    node *p;        //temporary node
    
    printf("\nEnter the item to be deleted :- ");
    scanf("%d",&key);

    if (head->number == key) {
        p = head->next;
        free(head);
        head = p;
    } else {
        preceding = find(head, key);
        if (preceding == NULL) {
            printf("\nKey not found!!\n");
        } else {
            p = preceding->next->next;
            free(preceding->next);
            preceding->next = p;
        }
    }
    return head;
}

node *find(node *p, int key) {
    if (p->next->number == key) {           //we know it is not the first so checking 2nd from head
        return p;
    } else {                                 //checking 3rd from head
        if (p->next->next == NULL) {
            return NULL;
        } else {
            find(p->next, key);              //recursive call
        }
    }
}

我尝试获取指针来保存新位置，但由于某种原因，它无法像插入函数那样工作，如下所示

new->next = head;
head = new;

Answer 1

据我了解，您正在返回 head，但没有将其设置在您的 main 方法中。你的意思是做这样的事情：

head = insert(head);
printf("\n");
print(head);
printf("\n");
head = delete(head);

也许您的 find 方法中需要返回：

return find(p->next, key);              //recursive call