题解 | #求二叉树的层序遍历#

import java.util.*;

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

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
        ArrayList<ArrayList<Integer>> treeList = new ArrayList<>();
        if(root == null){
            return treeList;
        } else {
            Queue<TreeNode> queue = new LinkedList<>();
            queue.offer(root);
            while(!queue.isEmpty()){
                ArrayList<Integer> list = new ArrayList<>();
                int size = queue.size();
                int count = 0;
                while(count<size){
                    TreeNode node = queue.poll();
                    list.add(node.val); // 出队一个元素
                    if(node.left!=null){
                        queue.offer(node.left);
                    }
                    if(node.right!=null){
                        queue.offer(node.right);
                    }
                    count++;
                }
                treeList.add(list);
            }
        }

        return treeList;


    }
}