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>> result = new ArrayList();
        if(root == null){
            return result;
        }
        Queue<TreeNode> q = new ArrayDeque<TreeNode>();
        q.add(root);
        while(!q.isEmpty()){
            ArrayList<Integer> layerList = new ArrayList<>();
            int n = q.size();
            
            for(int i = 0;i<n;i++){
                TreeNode node =q.poll();
                layerList.add(node.val);
                if(node.left!=null){
                    q.add(node.left);
                }
                if(node.right!=null){
                    q.add(node.right);
                }

            }
            result.add(layerList);
        }
        return result;
    }
}