leetcode——103.二叉树的锯齿形层次遍历

public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        if(root == null){
            return new ArrayList<>();
        }
        List<List<Integer>> results = new ArrayList<>();
        LinkedList<TreeNode> node_queue = new LinkedList<>();
        node_queue.addLast(root);
        node_queue.addLast(null);

        LinkedList<Integer> level_list = new LinkedList<>();
        boolean is_order_left = true;
        while(node_queue.size()>0){
            TreeNode curr_node = node_queue.pollFirst();
            if(curr_node != null){
                if(is_order_left){
                    level_list.addLast(curr_node.val);
                }else{
                    level_list.addFirst(curr_node.val);
                }
                if(curr_node.left!=null){
                    node_queue.addLast(curr_node.left);
                }
                if(curr_node.right!=null){
                    node_queue.addLast(curr_node.right);
                }
            }else{
                results.add(level_list);
                level_list = new LinkedList<>();
                if(node_queue.size()>0){
                    node_queue.addLast(null);
                }
                is_order_left = !is_order_left;
            }
        }
        return results;
    }

只是看懂了，不是自己写的，我好困啊，脑子有点转不动。。

——2020.7.1