145. Binary Tree Postorder Traversal - LeetCode Solution

class Solution:
    def postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        ans = []
        
        def trav(root):
            if not root:
                return 
            
            trav(root.left)
            trav(root.right)
        
            ans.append(root.val)
        
        trav(root)
        return ans