leetcode102_二叉树的层次遍历

public List<List<Integer>> levelOrder(TreeNode root) {
    List<List<Integer>> res = new ArrayList();
    if(root == null) return res;
    Deque<TreeNode> q = new LinkedList();
    q.offer(root);
    while(!q.isEmpty()) {
        int size = q.size();
        List<Integer> list = new ArrayList();
        while(size > 0) {
            TreeNode treeNode = q.poll();
            list.add(treeNode.val);
            if(treeNode.left != null) q.offer(treeNode.left);
            if(treeNode.right != null) q.offer(treeNode.right);
            size--;
        }
        res.add(list);
    }
    return res;
}