二叉树层次遍历

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>> list = new ArrayList<>();
        if(root == null) return list;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);

        while(!queue.isEmpty()){
            ArrayList<Integer> line = new ArrayList<>();
            int size = queue.size();
            for(int i = 0; i < size; ++i){
                TreeNode node = queue.poll();
                line.add(node.val);
                if(node.left != null){queue.add(node.left);}
                if(node.right != null){queue.add(node.right);}
            }
            list.add(line);
        }

        return list;
    }
}