二叉树的层序遍历 leetcode java

给你二叉树的根节点 root ，返回其节点值的 层序遍历 。 （即逐层地，从左到右访问所有节点）。 

 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */    
    public List> writeLevelOrder(TreeNode root) {
        List> retList = new ArrayList<>();
        if (root == null) {
            return retList;
        }
        Queue queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List ret = new ArrayList<>();
            while (size != 0) {
                TreeNode tmp = queue.poll();
                ret.add(tmp.val);
                if (tmp.left != null) {
                    queue.add(tmp.left);
                }
                if (tmp.right != null) {
                    queue.add(tmp.right);
                }
                size--;
            }
            retList.add(ret);
        }
        return retList;
    }