区块链如何查找无效的区块？

import hashlib

def calculate_hash(block):
    """计算区块的哈希值"""
    block_string = str(block['index']) + str(block['previous_hash']) + str(block['timestamp']) + str(block['transactions'])
    return hashlib.sha256(block_string.encode()).hexdigest()

def is_block_valid(block, previous_block):
    """验证区块的有效性"""
    if block['previous_hash'] != previous_block['hash']:
        return False
    if calculate_hash(block) != block['hash']:
        return False
    return True

# 示例区块数据
blockchain = [
    {
        'index': 0,
        'previous_hash': '0',
        'timestamp': 1633072800,
        'transactions': [],
        'hash': '000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f'
    },
    {
        'index': 1,
        'previous_hash': '000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f',
        'timestamp': 1633072860,
        'transactions': [{'from': 'A', 'to': 'B', 'amount': 10}],
        'hash': '816534932c2b7154836da6afc367695e6337db8a921823784c14378abed4f7d7'
    }
]

# 验证区块链的有效性
for i in range(1, len(blockchain)):
    if not is_block_valid(blockchain[i], blockchain[i-1]):
        print(f"区块 {blockchain[i]['index']} 无效")
    else:
        print(f"区块 {blockchain[i]['index']} 有效")