60 把二叉树打印成多行

题目描述
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。

public class Solution {
    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer> > arrayList = new ArrayList();
        if(pRoot == null)
            return arrayList;

        TreeNode last = pRoot;
        TreeNode nlast = null;
        Queue<TreeNode> queue = new LinkedList();
        ArrayList<Integer> list = new ArrayList();        

        queue.offer(pRoot);//竟然把这句忽略了

        while(!queue.isEmpty()){
            TreeNode cur = queue.poll();
            list.add(cur.val);
            if(cur.left != null){
                queue.offer(cur.left);
                nlast = cur.left;
            }
            if(cur.right != null){
                queue.offer(cur.right);
                nlast = cur.right;
            }
            if(cur == last){
                arrayList.add(list);
                list = new ArrayList();
                last = nlast;
            }                
        }

        return arrayList;
    }

}