In the absence of Malcolm and Donalbain, who will become King of Scotland in Macbeth?

After Malcolm and Donalbain run away to protect their own lives after Duncan's murder, Macbeth is named Duncan's heir and goes to Scone to be crowned. As the Thane of Glamis and Thane of Cawdor, Macbeth stands next in line for the throne. Duncan's sons are not only gone, they also are strongly suspected of having murdered their father.