input: Write a function to implement a function in Python that takes a Huffman tree and drop all its leaf nodes.
  
  
  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 test cases:
  assert dropLeafNodes(None) == None
  assert dropLeafNodes(TreeNode(1, TreeNode(2), TreeNode(3))) == TreeNode(1, TreeNode(2), TreeNode(3))
  assert dropLeafNodes(TreeNode(1, TreeNode(2, TreeNode(4), TreeNode(5)), TreeNode(3, TreeNode(6), TreeNode(7)))) == TreeNode(1, TreeNode(2), TreeNode(3))
  assert dropLeafNodes(TreeNode(1, TreeNode(2, TreeNode(4), None), TreeNode(3, None, TreeNode(5)))) == TreeNode(1, TreeNode(2), TreeNode(3))
  assert dropLeafNodes(TreeNode(1, TreeNode(2, None, TreeNode(4)), TreeNode(3, TreeNode(5), None))) == TreeNode(1, TreeNode(2), TreeNode(3))
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