用指针，结构体，文件，链表的知识写一个游戏注册登陆的C语言代码

以下是一个简单的游戏注册登录的C语言代码示例：

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

#define MAX_USERNAME_LENGTH 20
#define MAX_PASSWORD_LENGTH 20

typedef struct {
    char username[MAX_USERNAME_LENGTH];
    char password[MAX_PASSWORD_LENGTH];
} User;

typedef struct Node {
    User user;
    struct Node* next;
} Node;

Node* createUserNode(User user) {
    Node* newNode = (Node*)malloc(sizeof(Node));
    newNode->user = user;
    newNode->next = NULL;
    return newNode;
}

void addUser(Node** userList, User user) {
    Node* newNode = createUserNode(user);
    if (*userList == NULL) {
        *userList = newNode;
    } else {
        Node* current = *userList;
        while (current->next != NULL) {
            current = current->next;
        }
        current->next = newNode;
    }
}

int checkCredentials(Node* userList, char* username, char* password) {
    Node* current = userList;
    while (current != NULL) {
        if (strcmp(current->user.username, username) == 0 &&
            strcmp(current->user.password, password) == 0) {
            return 1; // 登录成功
        }
        current = current->next;
    }
    return 0; // 登录失败
}

void saveUsersToFile(Node* userList, char* filename) {
    FILE* file = fopen(filename, "w");
    if (file == NULL) {
        printf("无法打开文件 %s\n", filename);
        return;
    }

    Node* current = userList;
    while (current != NULL) {
        fprintf(file, "%s %s\n", current->user.username, current->user.password);
        current = current->next;
    }
    fclose(file);
}

Node* loadUsersFromFile(char* filename) {
    FILE* file = fopen(filename, "r");
    if (file == NULL) {
        printf("无法打开文件 %s\n", filename);
        return NULL;
    }

    Node* userList = NULL;
    char username[MAX_USERNAME_LENGTH];
    char password[MAX_PASSWORD_LENGTH];
    while (fscanf(file, "%s %s", username, password) == 2) {
        User user;
        strcpy(user.username, username);
        strcpy(user.password, password);
        addUser(&userList, user);
    }

    fclose(file);
    return userList;
}

void freeUserList(Node* userList) {
    Node* current = userList;
    while (current != NULL) {
        Node* temp = current;
        current = current->next;
        free(temp);
    }
}

int main() {
    Node* userList = NULL;

    // 从文件加载用户列表
    userList = loadUsersFromFile("users.txt");

    int choice;
    do {
        printf("1. 注册\n");
        printf("2. 登录\n");
        printf("3. 退出\n");
        printf("请选择: ");
        scanf("%d", &choice);

        if (choice == 1) {
            User newUser;
            printf("请输入用户名: ");
            scanf("%s", newUser.username);
            printf("请输入密码: ");
            scanf("%s", newUser.password);
            addUser(&userList, newUser);
            saveUsersToFile(userList, "users.txt");
            printf("注册成功！\n\n");
        } else if (choice == 2) {
            char username[MAX_USERNAME_LENGTH];
            char password[MAX_PASSWORD_LENGTH];
            printf("请输入用户名: ");
            scanf("%s", username);
            printf("请输入密码: ");
            scanf("%s", password);
            if (checkCredentials(userList, username, password)) {
                printf("登录成功！\n\n");
            } else {
                printf("用户名或密码错误！\n\n");
            }
        }
    } while (choice != 3);

    // 释放内存并保存用户列表到文件
    freeUserList(userList);
    saveUsersToFile(userList, "users.txt");

    return 0;
}

这段代码使用了指针、结构体、文件操作和链表等知识，实现了一个简单的游戏注册登录功能。用户可以选择注册新账号或登录已有账号，用户名和密码会保存在一个链表中，并可以将用户列表保存到文件中。