public static void main(String args[]){
        LevelOrderTraversal loi = new LevelOrderTraversal();
        BinaryTree bt = new BinaryTree();
        Node head = null;
        head = bt.addNode(10, head);
        head = bt.addNode(15, head);
        head = bt.addNode(5, head);
        head = bt.addNode(7, head);
        head = bt.addNode(19, head);
        head = bt.addNode(20, head);
        head = bt.addNode(-1, head);
        loi.levelOrder(head);
    }
Youtube link - https://youtu.be/9PHkM0Jri_4

http://www.geeksforgeeks.org/level-order-tree-traversal/ 
Test cases
Empty tree
Tree with only left side
Tree with only right side
Mixed size tree
Full tree
complete tree
src/com/interview/tree/LevelOrderTraversal.java