Swiftype is a external search engine that offers State-of-the-art indexing, custom result, ranking, synonyms, autocomplete, etc. Zendesk is beautifully simple customer service platform. It's possbile to replace the navtive Zendesk search with Swiftype, this will give you the awesome feature of autocomplete. The drawback is that Swiftype can only index public content on Zendesk, so if you have topics with any kind of restriction they will not indexed. The other drawback is that Swiftype doesn't respect the language settings so all languages are returned.
The nice thing is Swiftype provides code that with a bit of jQuery you can override the Zendesk search box. You can keep all the styles but the search will be from Swiftype. After you create account and set up a search engine goto the settings page select same page -> My website already has a search field -> my website needs a search results container.
In Zendesk Help Center configure theme on the Home page template drop in this code.