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# Writing a Binary Search Tree in Python – With Examples

Lane Wagner
Golang and javascript dev interested in distributed systems and cryptography
Originally published at qvault.io on ・6 min read

The post Writing a Binary Search Tree in Python – With Examples first appeared on Qvault.

A binary search tree, or BST for short, is a tree whose nodes each store a key greater than all their left child nodes and less than all of their right child nodes. Binary trees are useful for storing data in an organized way, which allows for it to be fetched, inserted, updated, and deleted quickly. The greater-than and less-than ordering of nodes mean that each comparison skips about half of the remaining tree, so the whole lookup takes time proportional to the number of nodes in the tree.

To be precise, binary search trees provide an average Big-O complexity of `O(log(n))` for retrieval, insertion, update, and delete operations. Log(n) is much faster than the linear `O(n)` time required to find items in an unsorted array. Many popular production databases such as PostgreSQL and MySQL use binary trees under the hood to speed up CRUD operations.

## Step 1 – BSTNode Class

Our implementation won’t use a `Tree` class, but instead just a `Node` class. Binary trees are really just a pointer to a root node that in turn connects to each child node, so we’ll run with that idea.

First, we create a constructor:

``````class BSTNode:
def  __init__ (self, val=None):
self.left = None
self.right = None
self.val = val
``````

We’ll allow a value (key) to be provided, but if one isn’t provided we’ll just set it to `None`. We’ll also initialize both children of the new node to `None`.

## Step 2 – Insert

We need a way to insert new data. The insert method is as follows:

``````def insert(self, val):
if not self.val:
self.val = val
return

if self.val == val:
return

if val < self.val:
if self.left:
self.left.insert(val)
return
self.left = BSTNode(val)
return

if self.right:
self.right.insert(val)
return
self.right = BSTNode(val)
``````

If the node doesn’t yet have a value, we can just set the given value and return. If we ever try to insert a value that also exists, we can also simply return as this can be considered a `noop`. If the given value is less than our node’s value and we already have a left child then we recursively call `insert` on our left child. If we don’t have a left child yet then we just make the given value our new left child. We can do the same (but inverted) for our right side.

## Step 3 – Get Min and Get Max

``````def get_min(self):
current = self
while current.left is not None:
current = current.left
return current.val

def get_max(self):
current = self
while current.right is not None:
current = current.right
return current.val
``````

`getMin` and `getMax` are useful helper functions, and they’re easy to write! They are simple recursive functions that traverse the edges of the tree to find the smallest or largest values stored therein.

## Step 4 – Delete

``````def delete(self, val):
if self == None:
return self
if val < self.val:
self.left = self.left.delete(val)
return self
if val > self.val:
self.right = self.right.delete(val)
return self
if self.right == None:
return self.left
if self.left == None:
return self.right
min_larger_node = self.right
while min_larger_node.left:
min_larger_node = min_larger_node.left
self.val = min_larger_node.val
self.right = self.right.delete(min_larger_node.val)
return selfdef delete(self, val):
if self == None:
return self
if val < self.val:
if self.left:
self.left = self.left.delete(val)
return self
if val > self.val:
if self.right:
self.right = self.right.delete(val)
return self
if self.right == None:
return self.left
if self.left == None:
return self.right
min_larger_node = self.right
while min_larger_node.left:
min_larger_node = min_larger_node.left
self.val = min_larger_node.val
self.right = self.right.delete(min_larger_node.val)
return selfdef delete(self, val):
if self == None:
return self
if val < self.val:
self.left = self.left.delete(val)
return self
if val > self.val:
self.right = self.right.delete(val)
return self
if self.right == None:
return self.left
if self.left == None:
return self.right
min_larger_node = self.right
while min_larger_node.left:
min_larger_node = min_larger_node.left
self.val = min_larger_node.val
self.right = self.right.delete(min_larger_node.val)
return self
``````

The delete operation is one of the more complex ones. It is a recursive function as well, but it also returns the new state of the given node after performing the delete operation. This allows a parent whose child has been deleted to properly set it’s `left` or `right` data member to `None`.

## Step 5 – Exists

The exists function is another simple recursive function that returns `True` or `False` depending on whether a given value already exists in the tree.

``````def exists(self, val):
if val == self.val:
return True

if val < self.val:
if self.left == None:
return False
return self.left.exists(val)

if self.right == None:
return False
return self.right.exists(val)
``````

## Step 6 – Inorder

It’s useful to be able to print out the tree in a readable format. The `inorder` method print’s the values in the tree in the order of their keys.

``````def inorder(self, vals):
if self.left is not None:
self.left.inorder(vals)
if self.val is not None:
vals.append(self.val)
if self.right is not None:
self.right.inorder(vals)
return vals
``````

## Step 7 – Preorder

``````def preorder(self, vals):
if self.val is not None:
vals.append(self.val)
if self.left is not None:
self.left.preorder(vals)
if self.right is not None:
self.right.preorder(vals)
return vals
``````

## Step 8 – Postorder

``````def postorder(self, vals):
if self.left is not None:
self.left.postorder(vals)
if self.right is not None:
self.right.postorder(vals)
if self.val is not None:
vals.append(self.val)
return vals
``````

## Usage

``````def main():
nums = [12, 6, 18, 19, 21, 11, 3, 5, 4, 24, 18]
bst = BSTNode()
for num in nums:
bst.insert(num)
print("preorder:")
print(bst.preorder([]))
print("#")

print("postorder:")
print(bst.postorder([]))
print("#")

print("inorder:")
print(bst.inorder([]))
print("#")

nums = [2, 6, 20]
print("deleting " + str(nums))
for num in nums:
bst.delete(num)
print("#")

print("4 exists:")
print(bst.exists(4))
print("2 exists:")
print(bst.exists(2))
print("12 exists:")
print(bst.exists(12))
print("18 exists:")
print(bst.exists(18))
``````

## Full Binary Search Tree in Python

``````class BSTNode:
def  __init__ (self, val=None):
self.left = None
self.right = None
self.val = val

def insert(self, val):
if not self.val:
self.val = val
return

if self.val == val:
return

if val < self.val:
if self.left:
self.left.insert(val)
return
self.left = BSTNode(val)
return

if self.right:
self.right.insert(val)
return
self.right = BSTNode(val)

def get_min(self):
current = self
while current.left is not None:
current = current.left
return current.val

def get_max(self):
current = self
while current.right is not None:
current = current.right
return current.val

def delete(self, val):
if self == None:
return self
if val < self.val:
if self.left:
self.left = self.left.delete(val)
return self
if val > self.val:
if self.right:
self.right = self.right.delete(val)
return self
if self.right == None:
return self.left
if self.left == None:
return self.right
min_larger_node = self.right
while min_larger_node.left:
min_larger_node = min_larger_node.left
self.val = min_larger_node.val
self.right = self.right.delete(min_larger_node.val)
return self

def exists(self, val):
if val == self.val:
return True

if val < self.val:
if self.left == None:
return False
return self.left.exists(val)

if self.right == None:
return False
return self.right.exists(val)

def preorder(self, vals):
if self.val is not None:
vals.append(self.val)
if self.left is not None:
self.left.preorder(vals)
if self.right is not None:
self.right.preorder(vals)
return vals

def inorder(self, vals):
if self.left is not None:
self.left.inorder(vals)
if self.val is not None:
vals.append(self.val)
if self.right is not None:
self.right.inorder(vals)
return vals

def postorder(self, vals):
if self.left is not None:
self.left.postorder(vals)
if self.right is not None:
self.right.postorder(vals)
if self.val is not None:
vals.append(self.val)
return vals
``````