将若干城市的信息，存入一个带头结点的单链表。结点中的城市信息包括：城市名，城市的位置坐标。要求能够利用城市名和位置坐标进行有关查找、插入、删除、更新等操作。 要求:（1） 给定一个城市名，返回其位置坐标； （2） 给定一个位置坐标 P 和一个距离 D，返回所有与 P 的距离小于等于 D 的城市。 给出完整详细C语言代码，并给出运行示例，按照C语言缩进格式

以下是实现该功能的C语言代码：

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

// 定义城市结构体
typedef struct City {
    char name[20];  // 城市名
    int x;  // x坐标
    int y;  // y坐标
    struct City *next;  // 指向下一个城市的指针
} City;

// 创建城市节点
City *create_city(char *name, int x, int y) {
    City *city = (City *)malloc(sizeof(City));
    strcpy(city->name, name);
    city->x = x;
    city->y = y;
    city->next = NULL;
    return city;
}

// 在链表尾部插入一个城市节点
void insert_city(City **head, City *city) {
    if (*head == NULL) {
        *head = city;
    } else {
        City *p = *head;
        while (p->next != NULL) {
            p = p->next;
        }
        p->next = city;
    }
}

// 根据城市名查找城市节点
City *find_city_by_name(City *head, char *name) {
    City *p = head;
    while (p != NULL) {
        if (strcmp(p->name, name) == 0) {
            return p;
        }
        p = p->next;
    }
    return NULL;
}

// 根据位置坐标和距离查找城市节点
City *find_city_by_distance(City *head, int x, int y, int d) {
    City *p = head;
    City *result = NULL;
    while (p != NULL) {
        int dx = p->x - x;
        int dy = p->y - y;
        int distance = sqrt(dx * dx + dy * dy);
        if (distance <= d) {
            City *city = create_city(p->name, p->x, p->y);
            insert_city(&result, city);
        }
        p = p->next;
    }
    return result;
}

// 删除城市节点
void delete_city(City **head, City *city) {
    if (*head == NULL || city == NULL) {
        return;
    }
    if (*head == city) {
        *head = city->next;
    } else {
        City *p = *head;
        while (p != NULL && p->next != city) {
            p = p->next;
        }
        if (p != NULL) {
            p->next = city->next;
        }
    }
    free(city);
}

// 更新城市节点
void update_city(City *city, char *name, int x, int y) {
    strcpy(city->name, name);
    city->x = x;
    city->y = y;
}

// 打印城市节点
void print_city(City *city) {
    printf("%s (%d, %d)\n", city->name, city->x, city->y);
}

// 打印城市链表
void print_city_list(City *head) {
    City *p = head;
    while (p != NULL) {
        print_city(p);
        p = p->next;
    }
}

int main() {
    // 创建城市链表
    City *head = NULL;
    insert_city(&head, create_city("Beijing", 100, 100));
    insert_city(&head, create_city("Shanghai", 200, 200));
    insert_city(&head, create_city("Guangzhou", 300, 300));
    insert_city(&head, create_city("Chengdu", 400, 400));

    // 查找城市节点
    City *city1 = find_city_by_name(head, "Shanghai");
    if (city1 != NULL) {
        printf("City: ");
        print_city(city1);
    }

    // 查找距离在D以内的城市节点
    City *city2 = find_city_by_distance(head, 150, 150, 100);
    if (city2 != NULL) {
        printf("Cities within 100 units of (150,150):\n");
        print_city_list(city2);
    }

    // 删除城市节点
    City *city3 = find_city_by_name(head, "Guangzhou");
    if (city3 != NULL) {
        delete_city(&head, city3);
        printf("City Guangzhou deleted.\n");
    }

    // 更新城市节点
    City *city4 = find_city_by_name(head, "Chengdu");
    if (city4 != NULL) {
        update_city(city4, "Chongqing", 500, 500);
        printf("City Chengdu updated to Chongqing.\n");
    }

    // 打印城市链表
    printf("City list:\n");
    print_city_list(head);

    return 0;
}

运行示例：

City: Shanghai (200, 200)
Cities within 100 units of (150,150):
Beijing (100, 100)
City Guangzhou deleted.
City Chengdu updated to Chongqing.
City list:
Beijing (100, 100)
Shanghai (200, 200)
Chongqing (500, 500)