剑指Offer刷题 把二叉树打印成多行

题目描述
从上到下按层打印二叉树，同一层结点从左至右输出。每一层输出一行。

来源：https://www.nowcoder.com/practice/445c44d982d04483b04a54f298796288?tpId=13&tqId=11213&tPage=3&rp=3&ru=%2Fta%2Fcoding-interviews&qru=%2Fta%2Fcoding-interviews%2Fquestion-ranking

1.非递归：利用队列

import java.util.ArrayList;
import java.util.LinkedList;

/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        if(pRoot==null){
            return new ArrayList<ArrayList<Integer>>();
        }
            LinkedList<TreeNode> queue = new LinkedList<>();
         ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        queue.offer(pRoot);
        while(!queue.isEmpty()){
            int size = queue.size();
            ArrayList<Integer> list = new ArrayList<>(size);
            for(int i = 0;i<size;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;
    }
    
}