按层打印二叉树--每行打印一层

 1 public void printTree(BinaryNode<T> root){
 2         if(root == null)
 3             return;
 4         Queue<BinaryNode<T>> queue = new LinkedList<>();
 5         
 6         int current;//当前层 还未打印的结点个数
 7         int next;//下一层结点个数
 8         
 9         queue.offer(root);
10         current = 1;
11         next = 0;
12         while(!queue.isEmpty()){
13             BinaryNode<T> currentNode = queue.poll();
14             System.out.printf("%-4d", currentNode.element);
15             current--;
16             
17             if(currentNode.left != null){
18                 queue.offer(currentNode.left);
19                 next++;
20             }
21             if(currentNode.right != null){
22                 queue.offer(currentNode.right);
23                 next++;
24             }
25             if(current ==0){
26                 System.out.println();
27                 current = next;
28                 next = 0;
29             }
30         }
31     }