题解 | 把二叉树打印成多行

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

/**
 * NC80 把二叉树打印成多行
 * @author d3y1
 */
public class Solution {
    private ArrayList<ArrayList<Integer>> lists = new ArrayList<>();

    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param pRoot TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> Print (TreeNode pRoot) {
        levelOrder(pRoot);
        return lists;
    }

    /**
     * 层序遍历
     * @param pRoot
     */
    private void levelOrder(TreeNode pRoot){
        if(pRoot == null){
            return;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(pRoot);

        int size;
        ArrayList<Integer> list;
        TreeNode node;
        while(!queue.isEmpty()){
            size = queue.size();
            list = new ArrayList<>();
            while(size-- > 0){
                node = queue.poll();
                list.add(node.val);
                if(node.left != null){
                    queue.offer(node.left);
                }
                if(node.right != null){
                    queue.offer(node.right);
                }
            }
            lists.add(list);
        }
    }
}