leetcode树之从上到下打印二叉树

从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行。例如:给定二叉树: [3,9,20,null,null,15,7],    3   /   9  20    /     15   7返回其层次遍历结果：[  [3],  [9,20],  [15,7]]提示：    节点总数 <= 1000注意：本题与主站 102 题相同：https://leetcode-cn.com/problems/binary-tree-level-order-traversal/来源：力扣（LeetCode）链接：https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-ii-lcof著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

/** * 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>> levelOrder(TreeNode root) {        if (root == null) {            return Collections.emptyList();        }        Queue queue = new LinkedList();        queue.offer(root);        List<List<Integer>> result = new ArrayList();        while (!queue.isEmpty()) {            List<Integer> list = new ArrayList<>();                int length = queue.size();                for (int i=0; i< length; i++) {                    TreeNode node = (TreeNode)queue.poll();                    if (node != null) {                        list.add(node.val);                        queue.offer(node.left);                        queue.offer(node.right);                    }                }            if (!list.isEmpty()){                result.add(list);              }          }        return result;    }}