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) {
        ArrayList<ArrayList<Integer>> ans = new ArrayList<ArrayList<Integer>>();
        // write code here
        if (root == null) {
            return ans;
        }

    
        Queue<TreeNode> q1 = new LinkedList<>();
        q1.add(root);
        while (!q1.isEmpty()) {
            int size = q1.size();
            ArrayList<Integer> curList = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode tmp = q1.poll();
                curList.add(tmp.val);

                if (tmp.left != null) {
                    q1.add(tmp.left);

                }

                if (tmp.right != null) {
                    q1.add(tmp.right);

                }

            }
            ans.add(curList);

        }




        return ans;
    }
}