思路

层序遍历 从左向右遍历每一层取最后一个数，代码如下：

public List<Integer> rightSideView(TreeNode root) {
        if (root==null){
            return new ArrayList<>();
        }
        Queue<TreeNode> queue = new LinkedList<>();
        List<Integer> list = new ArrayList<>();
        TreeNode cur=root;
        queue.add(cur);
      
        while (!queue.isEmpty()){
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                cur = queue.poll();
                if (i==size-1){
                    list.add(cur.val);
                }
                if (cur.left!=null){
                    queue.add(cur.left);
                }
                if (cur.right!=null){
                    queue.add(cur.right);
                }
            }
        }
        return list;
    }