题解 | #求二叉树的层序遍历#

import java.util.*;

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
 public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        Queue<Integer> level = new LinkedList<>();
        queue.add(root);
        level.add(1);
        int curLevel = level.peek();
        ArrayList<Integer> list = new ArrayList<>();
        while(!queue.isEmpty()){
            TreeNode poll = queue.poll();
            Integer levelPoll = level.poll();
            if(poll!=null&&levelPoll!=null){
                if(curLevel<levelPoll){
                    result.add(list);
                    list = new ArrayList<>();
                    curLevel = levelPoll;
                }
                list.add(poll.val);
                levelPoll++;
                queue.add(poll.left);
                level.add(levelPoll);
                queue.add(poll.right);
                level.add(levelPoll);
            }
        }
        result.add(list);
        return result;
    }
}