把二叉树打印成多行

从上到下按层打印二叉树，同一层结点从左至右输出。每一层输出一行。

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/* * 队列LinkedList完成层序遍历，用end记录每层结点数目 */ public class Solution {     ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {         ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();         if(pRoot == null){             return result;         }         Queue<TreeNode> layer = new LinkedList<TreeNode>();         ArrayList<Integer> layerList = new ArrayList<Integer>();         layer.add(pRoot);         int start = 0, end = 1;         while(!layer.isEmpty()){             TreeNode cur = layer.remove();             layerList.add(cur.val);             start++;             if(cur.left!=null){                 layer.add(cur.left);                        }             if(cur.right!=null){                 layer.add(cur.right);             }             if(start == end){                 end = layer.size();                 start = 0;                 result.add(layerList);                 layerList = new ArrayList<Integer>();             }         }         return result;     } }