199.二叉树的右视图

class Solution {
    List<Integer> result = new ArrayList<>();// 仔细读题，返回的是一维的，所以并不需要List<List<>>
    Queue<TreeNode> que = new LinkedList<>();

    public List<Integer> rightSideView(TreeNode root) {
        if(root != null) que.offer(root);
        while (!que.isEmpty()){
            int len = que.size();
            List<Integer> List = new ArrayList<>();
            
            for(int i = 0;i < len; i++){
                TreeNode node = que.poll();
                if (node.left != null) que.offer(node.left);// offer():添加
                if (node.right != null) que.offer(node.right);
                if (i == len - 1) result.add(node.val);
            }
        }
        return result;
    }
}