input: Write a python function to check if a value exists in the Binary Search Tree.
  assert bst.search(10) == True
  assert bst.search(5) == False
  assert bst.search(15) == True
output: class TreeNode:
      def __init__(self, key):
          self.left = None
          self.right = None
          self.val = key
  
  class BinarySearchTree:
      def __init__(self):
          self.root = None
  
      def insert(self, key):
          if self.root is None:
              self.root = TreeNode(key)
          else:
              self._insert(self.root, key)
  
      def _insert(self, node, key):
          if key < node.val:
              if node.left is None:
                  node.left = TreeNode(key)
              else:
                  self._insert(node.left, key)
          else:
              if node.right is None:
                  node.right = TreeNode(key)
              else:
                  self._insert(node.right, key)
  
      def search(self, key):
          return self._search(self.root, key)
  
      def _search(self, node, key):
          if node is None:
              return False
          if node.val == key:
              return True
          elif key < node.val:
              return self._search(node.left, key)
          else:
              return self._search(node.right, key)
  
  bst = BinarySearchTree()
  bst.insert(10)
  bst.insert(15)
  bst.insert(8)