从上到下打印出二叉树的每个节点，同一层的节点按照从左到右的顺序打印。
例如:
给定二叉树: [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
返回：
[3,9,20,15,7]
/*** Definition for a binary tree node.* public class TreeNode {* int val;* TreeNode left;* TreeNode right;* TreeNode(int x) { val = x; }* }*/
class Solution {public int[] levelOrder(TreeNode root) {//用队列保存根节点，用int[]记录返回的数组if(root == null) return new int[0];ArrayList<Integer> arr = new ArrayList<>();Queue<TreeNode> queue = new LinkedList<>();queue.add(root);while(!queue.isEmpty()) {//根节点TreeNode r = queue.poll();arr.add(r.val);//左子树if(r.left != null) queue.add(r.left);//右子树if(r.right != null) queue.add(r.right);}int[] res = new int[arr.size()];for(int i=0; i<arr.size();i++) {res[i] = arr.get(i);}return res;}
}