leetcode-中等-队列-二叉树的层次遍历

public List<List<Integer>> levelOrder(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        List<List<Integer>> res =new LinkedList<>();
        if(root != null) {
            queue.offer(root);
        }
        while(!queue.isEmpty()){
            int len = queue.size();
            LinkedList<Integer> temp = new LinkedList<>();
            for(int i = 0; i < len; i++){
                TreeNode curr = queue.remove();
                temp.add(curr.val);
                if(curr.left != null){
                    queue.offer(curr.left);
                }
                if(curr.right != null){
                    queue.offer(curr.right);
                }
            }
            res.add(temp);
        }
        return res;
    }