PopulateLevelPointers

class ListTreeNode{
    int val;
    ListTreeNode left;
    ListTreeNode right;
    ListTreeNode next;
 
    public ListTreeNode(int val) {
        this.val = val;
        this.left = null;
        this.right = null;
        this.next = null;
    }
}
 
public class PopulateLevelPointers {
    public ListTreeNode connect (ListTreeNode root){
        if ( root == null){
            return  null;
        }
        ListTreeNode currentNode = root;// Start at the root node
        while (currentNode.left != null){ // Traverse each level of the tree until there are no more levels
            ListTreeNode level = currentNode;// Set the starting node of the level to the current node
            while (level != null){ // Traverse each node of the level
                level.left.next = level.right;// Connect the left child node to the right child node
                if ( level.next != null){
                    level.right.next = level.next.left; // Connect the right child node to the left child node of the next node on the same level
                }
                level = level.next;// Move to the next node on the same level
            }
            currentNode = currentNode.left;// Move to the next level of the tree
        }
        return root;// Return the modified tree
    }
 
    public static void main(String[] args) {
        ListTreeNode root = new ListTreeNode(1);
        root.left = new ListTreeNode(2);
        root.right = new ListTreeNode(3);
        root.left.left = new ListTreeNode(4);
        root.left.right = new ListTreeNode(5);
        root.right.left = new ListTreeNode(6);
        root.right.right = new ListTreeNode(7);
 
        PopulateLevelPointers plp = new PopulateLevelPointers();
        ListTreeNode result = plp.connect(root);
 
        ListTreeNode current = result;
        while (current != null){
            ListTreeNode level = current;
            while (level != null){
                System.out.print(level.val + " -> ");
                level = level.next;
            }
            System.out.println("x");
            current = current.left;
        }
    }
}