求二叉树层序遍历

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