[leetcode] 94. Binary Tree Inorder Traversal

이진 트리의 inorder traversal 

class Solution:
    def dfs(self, root, answer):
        if root is None:
            return

        self.dfs(root.left, answer)
        answer.append(root.val)
        self.dfs(root.right, answer)

    def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        answer = []
        self.dfs(root, answer)
        return answer