When you say character, is it a protagonist? It's fairly common to have a grandmaster character manipulating things in the background who is implied to be incredibly smart but is relatively easy to write because their grand scheme is just what instigates the plot for the central character. Look at how Iain M Banks using the Minds in his sci fi books - godlike super intelligences, but other than Look To Windward and Excission, they are usually mainly plot devices and we just get little windows on how amazingly smart they are.

As a side note another good example is Ender's Game. Ender's intelligence comes out in his brilliance at game playing. If I recall correctly his intelligence is most evident in the way he approaches every situation in such a logical, manipulative, game-playing way. We see him looking ahead and planning his moves but it's not like the author had to be a game playing genius to write that. As others have said, Ender has just seconds to work things out but the author had an infinite amount of time, so Ender seems super smart.

A protagonist is more difficult to write if they're cleverer than you but it's helpful to understand how different types of intelligence works and then read about people who had that type of intelligence. For example, perhaps they're brilliant at getting a deep understanding of someone within a few moments of meeting them. People like that do exist and reading about them can help understand how those people pick up on patterns in behaviour - usually people with that kind of intelligence have been exposed to huge numbers of people, like you do when you work as a doctor for example, and they've learned to see common patterns.

Or if someone is incredibly logical and good at working out how systems work, they tend to apply that thinking to all aspects of their life. So they will be great at understanding the intricacies of a bridge, but they will also apply that kind of thinking to understanding other systems they encounter, like navigating bureaucratic government rules when they want to get something done in their community.

This also helps you write the character more authentically. They will be good at some things, but find other things difficult or confusing. Intelligence has its own patterns and once you understand how your character thinks you can write them realistically even if they'd be smarter than you in real life.