Leetcode：剑指 Offer 32 - III. 从上到下打印二叉树 III

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List> levelOrder(TreeNode root) {
        List> res=new ArrayList>();
        if(root==null){
            return res;
        }
        Queue queue=new LinkedList();
        queue.offer(root);
        while(!queue.isEmpty()){
            //使用LinkedList链表可以选择进行头插法还是尾插法，方便旋转
            LinkedList level=new LinkedList();
            int queuesize=queue.size();
            for(int i=0;i