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 TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here

        ArrayList<ArrayList<Integer>> array = new ArrayList<>();

        if (root == null) {
            return array;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);

        while (!queue.isEmpty()) {
            int size = queue.size();

            ArrayList<Integer> list = new ArrayList<>();
            array.add(list);

            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();

                if (node != null) {
                    list.add(node.val);

                    if (node.left != null) {
                        queue.add(node.left);
                    }

                    if (node.right != null) {
                        queue.add(node.right);
                    }
                }
            }
        }

        return array;
    }


}