How do I fix the error LNK2005 struct node * root" (?root@@3PAUnode@@A) already defined in AVL.obj?

Question

My assignment is separated into a header file (.h), implementation file (.cpp) and main file (*.cpp).

The program is supposed to do the following:

Build an AVL tree and display it on the screen at any time during the insertion. After the tree was completely built and balanced, perform: Inorder traversal, Preorder traversal, and Postorder traversal.

My code is as follows:

AVL.h (header file)

#ifndef AVL_H_
#define AVL_H_

struct node
{
    int data;
    struct node *left;
    struct node *right;
}*root;


class AVL
{
public:
    int height(node *);
    int difference(node *);
    node *rotateRight(node *);
    node *rotateLeft(node *);
    node *rotateLR(node *);
    node *rotateRL(node *);
    node *balance(node *);
    node *insert(node *, int);
    void displayAVL(node *, int);
    void inOrder(node *);
    void preOrder(node *);
    void postOrder(node *);
    AVL(){root = NULL;}
};

#endif

AVL.cpp (implementation file)

#include <iostream>
#include<cstdio>
#include<sstream>
#include<algorithm>

#include "AVL.h"
#define pow2(n)(1<<(n))
using namespace std;


int AVL::height(node *hold)
{
    int ht;
    if(hold != NULL)
    {
        int leftHt = height(hold->left);
        int rightHt = height(hold->right);
        int max_Ht = max(leftHt, rightHt);
        ht = max_Ht + 1;
    }

    return ht;
}

int AVL::difference(node *hold)
{
    int leftHt = height(hold->left);
    int rightHt = height(hold->right);
    int b_factor = leftHt - rightHt;

    return b_factor;
}

node *AVL::rotateRight(node *parent)
{
    node *hold;
    hold = parent->right;
    parent->right = hold->left;
    hold->left = parent;

    return hold;

}

node *AVL::rotateLeft(node *parent)
{
    node *hold;
    hold = parent->left;
    parent->left = hold->right;
    hold->right = parent;

    return hold;
}

node *AVL::rotateLR(node *parent)
{
    node *hold;
    hold = parent->left;
    parent->left = rotateRight(hold);

    return rotateLeft(parent);
}

node *AVL::rotateRL(node *parent)
{
    node *hold;
    hold = parent->right;
    parent->left = rotateLeft(hold);

    return rotateRight(parent);
}

node *AVL::balance(node *hold)
{
    int b_factor = difference(hold);

    if(b_factor > 1)
    {
        if(difference(hold->left) > 0)
            hold = rotateLeft(hold);
        else
            hold = rotateLR(hold);
    }
    else if(b_factor < -1)
    {
        if(difference(hold->right) > 0)
            hold = rotateRL(hold);
        else
            hold = rotateRight(hold);
    }

    return hold;
}

node *AVL::insert(node *root, int value)
{
    if(root == NULL)
    {
        root = new node;
        root->data = value;
        root->left = NULL;
        root->right = NULL;
        return root;
    }
    else if(value < root->data)
    {
        root->left = insert(root->left, value);
        root = balance(root);
    }
    else if(value >= root->data)
    {
        root->right = insert(root->right, value);
        root = balance(root);
    }

    return root;
}

void AVL::displayAVL(node *pointer, int level)
{
    if(pointer != NULL)
    {
        displayAVL(pointer->right, level + 1);
        cout<< "\n";
        if(pointer == root)
            cout<<"Root -> ";
        for(int i = 0; i < level&&pointer != root; i++)
        {
            cout<<" ";
        }
        cout<< pointer->data;
        displayAVL(pointer->left, level + 1);
    }
}

void AVL::inOrder(node *tree)
{
    if(tree == NULL)
        return;

    inOrder(tree->left);
    cout<<tree->data<<" ";
    inOrder(tree->right);
}

void AVL::preOrder(node *tree)
{
    if(tree == NULL)
        return;

    cout<<tree->data<<" ";
    preOrder(tree->left);
    preOrder(tree->right);
}

void AVL::postOrder(node *tree)
{
    if(tree == NULL)
        return;

    postOrder(tree->left);
    postOrder(tree->right);
    cout<<tree->data<<" ";
}

Exercise_4_main.cpp (main file)

#include <iostream>
#include "AVL.h"

using namespace std;

int main()
{
    int choice;
    int value;
    AVL avlTree;

    cout<< "-----------------------\n";
    cout<< "AVL Tree Implementation\n";
    cout<< "-----------------------\n";
    cout<< "1. Insert an element into the tree\n";
    cout<< "2. Display Balanced AVL Tree\n";
    cout<< "3. InOrder tranversal\n";
    cout<< "4. PreOrder tranversal\n";
    cout<< "5. PostOrder tranversal\n";
    cout<< "6. Exit";
    cout<< "Enter your choice: ";
    cin>> choice;

    while(choice != 6)
    {
        if(choice == 1)
        {
            cout<<"Enter value to be inserted: ";
            cin>> value;
            root = avlTree.insert(root, value);
        }
        else if(choice == 2)
        {
            if(root == NULL)
            {
                cout<< "Tree is Empty\n";
            }
            cout<< "Balanced AVL Tree:\n";
            avlTree.displayAVL(root, 1);
        }
        else if(choice == 3)
        {
            cout<< "Inorder Tranversal:\n";
            avlTree.inOrder(root);
            cout<<"\n";
        }
        else if(choice == 4)
        {
            cout<< "Preorder Tranversal:\n";
            avlTree.preOrder(root);
            cout<<"\n";
        }
        else
            cout<< "Postorder Tranversal:\n";
            avlTree.postOrder(root);
            cout<< "\n";

        cout<< "-----------------------\n";
        cout<< "AVL Tree Implementation\n";
        cout<< "-----------------------\n";
        cout<< "1. Insert an element into the tree\n";
        cout<< "2. Display Balanced AVL Tree\n";
        cout<< "3. InOrder tranversal\n";
        cout<< "4. PreOrder tranversal\n";
        cout<< "5. PostOrder tranversal\n";
        cout<< "6. Exit";
        cout<< "Enter your choice: ";
        cin>> choice;
    }

    system("pause");
    return 0;
}

I compiled my code and I am getting the error LNK2005: "struct node * root" (?root@@3PAUnode@@A) already defined in AVL.obj. Can anyone help me figure out how to fix this problem?