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>> list = new ArrayList<ArrayList<Integer>>();
        if(root == null){
            return list;
        }
        
        Queue<TreeNode> queue = new LinkedList();
        queue.add(root);
        while(!queue.isEmpty()){
            Queue<TreeNode> nextQueue = new LinkedList();
            ArrayList<Integer> subList = new ArrayList<Integer>();
            while(!queue.isEmpty()){
                TreeNode node = queue.poll();
                subList.add(node.val);
                
                if(node.left != null){
                    nextQueue.add(node.left);
                }
                if(node.right != null){
                    nextQueue.add(node.right);
                }
            }
            
            list.add(subList);
            
            while(!nextQueue.isEmpty()){
                queue.add(nextQueue.poll());
            }
        }
        
        return list;
    }
}