94. Binary Tree Inorder Traversal

https://leetcode.com/problems/binary-tree-inorder-traversal

class Solution:
    def postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        return self.travel([], root)
    def travel(self, result, node):
        if not node:
            return

        self.travel(result, node.left)
        result.append(node.val)
        self.travel(result, node.right)
        return result