input: Write a function to drop all leaf nodes from a given Huffman tree. If a node does not have any children, it should be removed from the tree. The function should be recursive and should return the modified tree. If the input tree is empty or does not exist, the function should return None.
  
  def dropLeafNodes(head): 
      
      # 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
  
  assert dropLeafNodes(None)==None
  assert dropLeafNodes(TreeNode(1, TreeNode(2, TreeNode(4), TreeNode(5)), TreeNode(3, TreeNode(6), TreeNode(7)))).left.left==None
  assert dropLeafNodes(TreeNode(1, TreeNode(2, TreeNode(4), None), None)).left.left==None
  assert dropLeafNodes(TreeNode(1, None, None))==None
output: def dropLeafNodes(head): 
      
      # 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