二叉树的层次遍历

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