GENERAL IDEA
1. Find one or more base cases
2. Call the same function on the left subtree
3. Call the same function on the right subtree
4. Join the results

sum of elements

def sum_elements(node):
	if not node:
		return 0
	leftSum = sum_elements(node.left)
	rightSum = sum_elements(node.right)
	return node.val + leftSum + rightSum

maximum element

def max_element(node):
	if not node:
		return float("-inf")
	leftMax = max_element(node.left)
	rightMax = max_element(node.right)
	return max(node.val, leftMax, rightMax)

tree height

def get_height(node):
	if not node:
		return 0
	leftHeight = get_height(node.left)
	rightHeight = get_height(node.right)
	return 1 + max(leftHeight, rightHeight)

exists in tree

def existsInTree(node, val):
	if not node:
		return False
	else:
		inLeft = existsInTree(root.left, value)
		inRight = existsInTree(root.right, value)
		return node.val == val or inLeft or inRight