题解 | #把二叉树打印成多行#

双队列实现，第一层打印完存第二层，第二层打印时存入第三层

import java.util.ArrayList;


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

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

    }

}
*/
import java.util.*;
public class Solution {
    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        TreeNode root = pRoot; 
        Queue<TreeNode> queue = new LinkedList<>();
        Queue<TreeNode> queue2 = new LinkedList<>();
        ArrayList<ArrayList<Integer> > list = new ArrayList<>();
        TreeNode op;
        queue.offer(root);
        if(root == null){
            return list;
        }
        while(true){
            List<Integer> li = new ArrayList<>();

            while(!queue.isEmpty()){
                op = queue.poll();
                if(op.left != null){
                    queue2.offer(op.left);
                }
                if(op.right != null){
                    queue2.offer(op.right);
                }
                li.add(op.val);
            }
            if(li.size() != 0){
                list.add(new ArrayList(li));
            }
            li.clear();
            while(!queue2.isEmpty()){
                op = queue2.poll();
                if(op.left != null){
                    queue.offer(op.left);
                }
                if(op.right != null){
                    queue.offer(op.right);
                }
                li.add(op.val);
            } 
            if(li.size() != 0){
                list.add(new ArrayList(li));
            }
            li.clear();
            if(queue.isEmpty() && queue2.isEmpty()){
                break;
            }
        }
        return list;
    }
    
}