how to make a variable local

Question

I made this recursive method that calculates the longest path in a binary tree. the path its store in an arralist and then returned. however, i had to declare the array list variable global. is it possible to make this method but his the array list variable being local.

public static <T> ArrayList<T> longestPath(BinaryNode<T> root){
    //ArrayList path = new ArrayList();

   if(root == null) return null;

   if(height(root.left) > height(root.right)){
       path.add(root.element);

       longestPath(root.left);


   }else{
       path.add(root.element);

       longestPath(root.right);

   }

   return path;

}

The reason i had to make it global is because its a recursive program and every time it call itself it will create a new array list object variable with difference address if you know what i mean.

Answer 1

Pass the arraylist in the method parameters:

public static <T> List<T> longestPath(BinaryNode<T> root, List<T> path){

Then when you do the recursive call:

longestPath(root.right, path);

Just pass a new Arraylist() when you call the method initially

Answer 2

What you should do is have your main function create an ArrayList and pass it into a helper function that does all the work; e.g:

public static ArrayList longestPath(BinaryNode root)
{
    ArrayList path = new ArrayList();
    return longestPathHelper(root, path);
}

private static ArrayList longestPathHelper(BinaryNode root, ArrayList path)
{
    // Existing code, except recursive calls pass path as well
}

Answer 3

If you need access to the variable and cannot make it global, your other option is to pass it as a parameter:

    public static <T> ArrayList<T> longestPath(BinaryNode<T> root, ArrayList path) {
    //...

Answer 4

if you pass the ArrayList to your recursive function yes:

public static <T> ArrayList<T> longestPath(BinaryNode<T> root, ArrayList path){
ArrayList lPath = path;

 if(root == null) return null;

 if(height(root.left) > height(root.right)){
   lPath.add(root.element);

   longestPath(root.left, lPath);


 }else{
   lPath.add(root.element);

   longestPath(root.right, lPath);

 }

 return lPath;

 }