leetcode binary-tree-level-order-traversal输出树的每一层

题意：
给定二叉树，返回其节点值的级别顺序遍历。
For example:
Given binary tree{3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        LinkedList<TreeNode> que = new LinkedList<TreeNode>();
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        if(root==null)return res;
        que.add(root);
        while(!que.isEmpty()){
            int size = que.size();
            ArrayList<Integer> list = new ArrayList<Integer>();
            for(int i=0;i<size;i++){
                TreeNode cur = que.pop();
                list.add(cur.val);
                if(cur.left!=null){
                    que.add(cur.left);
                }
                if(cur.right!=null){
                    que.add(cur.right);
                }
            }
            res.add(list);
        }
        return res;
    }
}