剑指 Offer 32 - II. 从上到下打印二叉树 II

/**
 * 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) {
        //结果集res
        List<List<Integer>> res = new ArrayList<>();
        if(root == null){//判空
           return res;
        }
        Queue<TreeNode> queue = new LinkedList();//存放TreeNode 队列
        queue.add(root);//根节点入队
        while(!queue.isEmpty()){//遍历队列queue
            ArrayList<Integer> ans = new ArrayList<>();//定义一个list，装同一层的TreeNode
            for(int i = queue.size();i > 0;i--){
                TreeNode node = queue.poll();
                ans.add(node.val);
                if(node.left != null){
                queue.add(node.left);
                }
                if(node.right != null){
                queue.add(node.right);
                }
            }
            res.add(ans);
        }
        return res;
    }
}