input: from typing import Optional
  
  class TreeNode:
      def __init__(self, val=0, left=None, right=None):
          self.val = val
          self.left = left
          self.right = right
  
  def dropLeafNodes(head: Optional[TreeNode]) -> Optional[TreeNode]: 
      """
      Drop all leaf nodes from the given Huffman tree.
  
      >>> head = TreeNode(1, TreeNode(2, TreeNode(4), TreeNode(5)), TreeNode(3, TreeNode(6), TreeNode(7)))
      >>> result = dropLeafNodes(head)
      >>> result.val
      1
      >>> result.left.val
      2
      >>> result.right.val
      3
      >>> result.left.left is None and result.left.right is None and result.right.left is None and result.right.right is None
      True
      """
      
      # If node is not None and doesn't have any children 
      if head and not head.left and not head.right: 
          return None
    
      # Don't change anything if head is None 
      if not head: 
          return None
    
      # Recursively call dropLeafNodes on left and right subtrees
      head.left = dropLeafNodes(head.left) 
      head.right = dropLeafNodes(head.right) 
    
      # Return head of the modified tree
      return head
output:     # If node is not None and doesn't have any children 
      if head and not head.left and not head.right: 
          return None
    
      # Don't change anything if head is None 
      if not head: 
          return None
    
      # Recursively call dropLeafNodes on left and right subtrees
      head.left = dropLeafNodes(head.left) 
      head.right = dropLeafNodes(head.right) 
    
      # Return head of the modified tree
      return head