106. Construct Binary Tree from Inorder and Postorder Traversal

class Solution:
    def buildTree(self, inorder: List[int], postorder: List[int]) -> TreeNode:
        def helper(in_left, in_right):
            # if there is no elements to construct subtrees
            if in_left > in_right:
                return None
            
            val = postorder.pop()
            root = TreeNode(val)

            index = idx_map[val]
 
            root.right = helper(index + 1, in_right)
            root.left = helper(in_left, index - 1)
            return root
        
        idx_map = {val:idx for idx, val in enumerate(inorder)} 
        return helper(0, len(inorder) - 1)