public boolean isComplete(Node root){
        Queue<Node> queue = new LinkedList<Node>();
        queue.offer(root);
        boolean foundFirstNonFull = false;
        while(!queue.isEmpty()){
            root = queue.poll();
            if(foundFirstNonFull){
                if(root.left != null || root.right != null){
                    return false;
                }
                continue;
            }
            if(root.left != null && root.right != null){
                queue.offer(root.left);
                queue.offer(root.right);
            }else if(root.left != null){
                queue.offer(root.left);
                foundFirstNonFull = true;
            }else if(root.right != null){
                return false;
            }else{
                foundFirstNonFull = true;
            }
        }
        return true;
    }
http://www.geeksforgeeks.org/check-if-a-given-binary-tree-is-complete-tree-or-not/
Test cases:
A node with only right child
A node with only left child
A node with both left and right child
src/com/interview/tree/IsCompleteBinaryTree.java