包含层次关系的二叉树的层次遍历结果

题目链接：https://www.nowcoder.com/questionTerminal/04a5560e43e24e9db4595865dc9c63a3?answerType=1&f=discussion
来源：牛客网

解题思路
由层次遍历的单层循环，变为双层循环，内层循环每次遍历一层

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;

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