Hashgraph public ledger (51% attack)
Seeing this a year late, but if this is still a question then -

Hashgraph isn't exactly a voting based algorithm. It does "virtual voting" through "gossip about gossip" i.e. nodes not just gossip that there is a new transaction, but also gossip about when they heard it, who told them/how they heard it (literally gossiping about the gossip!)
This way every node has the COMPLETE knowledge of the network. If node A knows everything that node B knows, A can also find out what B will vote (WITHOUT B actually voting!)

Also, hashgraph never says that 51% attack can't occur. Through Byzantine Fault Tolerance even a  34% attack can occur (if more than 1/3rd of the nodes are malicious).
Hashgraph just says that DDoS or DoS (Denial of Service) attacks can't occur because there are no leaders.

Refer to the Hashgraph whitepaper or the scientific paper on its consensus algorithm: https://docs.hedera.com/docs/hashgraph-overview or https://www.hedera.com/whitepaper