数据结构：树🌲

时间复杂度：O(n)

空间复杂度：O(n)

代码实现：

class Solution:
    def rightSideView(self, root: Optional[TreeNode]) -> List[int]:
        if not root: return []
        q = [root]
        res = []
        while q:
            out = None
            for _ in range(len(q)):
                out = q.pop(0)
                if out.left:
                    q.append(out.left)
                if out.right:
                    q.append(out.right)
            res.append(out.val)
        
        return res