【常见面试算法】二叉树层次打印
从上到下按层打印二叉树，同一层的结点按从左到右的顺序打印，每一层打印到一行。
样例
输入如下图所示二叉树[8, 12, 2, null, null, 6, null, 4, null, null, null] 8 / \ 12 2 / 6 / 4 输出：[[8], [12, 2], [6], [4]]
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<List<Integer>> printFromTopToBottom(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
if (root == null) {
return res;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while (!queue.isEmpty()) {
int len = queue.size();
List<Integer> list = new ArrayList<>();
for (int i = 0; i < len; i++) {
TreeNode node = queue.poll();
list.add(node.val);
if (node.left != null) queue.offer(node.left);
if (node.right != null) queue.offer(node.right);
}
res.add(list);
}
return res;
}
} 
