使用C++类定义多个数据结构

class ListNode {
public:
    int val;
    ListNode* next;
    ListNode(int x) : val(x), next(nullptr) {}
};

class LinkedList {
public:
    ListNode* head;
    LinkedList() : head(nullptr) {}
};

class Stack {
private:
    vector<int> data;
public:
    void push(int x) {
        data.push_back(x);
    }
    void pop() {
        if (!empty()) {
            data.pop_back();
        }
    }
    int top() {
        return data.back();
    }
    bool empty() {
        return data.empty();
    }
};

class Queue {
private:
    vector<int> data;
public:
    void push(int x) {
        data.push_back(x);
    }
    void pop() {
        if (!empty()) {
            data.erase(data.begin());
        }
    }
    int front() {
        return data.front();
    }
    bool empty() {
        return data.empty();
    }
};

class TreeNode {
public:
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};

class Tree {
public:
    TreeNode* root;
    Tree() : root(nullptr) {}
};