input: Write a function named build_blockchain to build a simple blockchain system in Python. The function should define an instance of the Block class to create the hash of the block contents, create the blockchain by adding blocks, and print the blockchain. The function should not take any input.
  assert build_blockchain() is not None
output: def build_blockchain():
      import hashlib
      import time
  
      class Block:
          def __init__(self, index, previous_hash, data, timestamp):
              self.index = index
              self.previous_hash = previous_hash
              self.data = data
              self.timestamp = timestamp
              self.hash = self.calculate_hash()
  
          def calculate_hash(self):
              block_string = f"{self.index}{self.previous_hash}{self.data}{self.timestamp}"
              return hashlib.sha256(block_string.encode()).hexdigest()
  
      def create_genesis_block():
          return Block(0, "0", "Genesis Block", time.time())
  
      blockchain = [create_genesis_block()]
  
      def add_block(data):
          previous_block = blockchain[-1]
          new_index = previous_block.index + 1
          new_timestamp = time.time()
          new_block = Block(new_index, previous_block.hash, data, new_timestamp)
          blockchain.append(new_block)
  
      add_block("First Block")
      add_block("Second Block")
  
      return blockchain