Insertion

Inserting in a BST is straight forward

def insertIntoBST(root, val):
	if root is None:
		return TreeNode(val)
	if val < root.val:
		root.left = self.insertIntoBST(root.left, val)
	else:
		root.right = self.insertIntoBST(root.right, val)
	return root

Deletion

In deletion to reduce complexity, we exchange the node to be deleted with it's Inorder predecessor, and remove the predecessor, this make it easy since predecessor is usually a leaf node.

def helper(root, key):
	if root is not None:
		if root.val == key:
			if root.left and root.right:
				pre = root.left
					while pre.right:
						pre = pre.right
					# Assign predecessor as root 
					root.val = pre.val
					# Delete the predecessor
					root.left = helper(root.left, pre.val)
			elif root.left or root.right:
				return root.left if root.left else root.right
			else:
				return None
		elif key < root.val:
			root.left = helper(root.left, key)
		else:
			root.right = helper(root.right, key)
	return root