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
        if(root == null){
            return null;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        ArrayList<ArrayList<Integer>> ans = new ArrayList<>();
        queue.add(root);
        while(!queue.isEmpty()){
            int size = queue.size();
            for(int i = 0;i<size;i++){
                TreeNode cur = queue.poll();
                list.add(cur.val);
                if(cur.left!=null) queue.add(cur.left);
                if(cur.right!=null) queue.add(cur.right);
            }
            ans.add(list);
        }
        return ans;
    }
}