Data Structures and Algorithms in Python/

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Solution Review: Checking the BST property

Solution Review: Checking the BST property

This lesson contains the solution review for ​Checking the BST property challenge.

We'll cover the following...

Recall the in-order traversal that we learned in the Binary Trees chapter. The in-order traversal of a Binary Search Tree gives us the list of nodes in sorted order.

In the code widget below, we have implemented the in-order traversal for BST and you can confirm the traversal in the illustration above.

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class Node(object):
  def __init__(self, data):
    self.data = data
    self.left = None
    self.right = None
class BST(object):
    def __init__(self, root):
        self.root = Node(root)
    def insert(self, data):
        if self.root is None:
            self.root = Node(data)
        else:
            self._insert(data, self.root)
    def _insert(self, data, cur_node):
        if data < cur_node.data:
            if cur_node.left is None:
                cur_node.left = Node(data)
            else:
                self._insert(data, cur_node.left)
        elif data > cur_node.data:
            if cur_node.right is None:
                cur_node.right = Node(data)
            else:
                self._insert(data, cur_node.right)
        else:
            print("Value already in tree!")
    def inorder_print_tree(self):
        if self.root:
            self._inorder_print_tree(self.root)
    def _inorder_print_tree(self, cur_node):
        if cur_node:
            self._inorder_print_tree(cur_node.left)
            print(str(cur_node.data))
            self._inorder_print_tree(cur_node.right)
    def find(self, data):
        if self.root:
            is_found = self._find(data, self.root)
            if is_found:
                return True
            return False
        else:
            return None
    def _find(self, data, cur_node):
        if data > cur_node.data and cur_node.right:
            return self._find(data, cur_node.right)
        elif data < cur_node.data and cur_node.left:
            return self._find(data, cur_node.left)
        if data == cur_node.data:
            return True
bst = BST(7)
bst.insert(3)
bst.insert(10)
bst.insert(5)
bst.insert(1)
bst.insert(8)
bst.insert(9)
bst.insert(2)
bst.inorder_print_tree()

We have discussed in-order traversal above because we’ll be using a similar idea to check whether a tree satisfies the BST property or not. If we traverse a binary tree in-order and it results in a sorted list, then the tree satisfies the BST property.

Implementation

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