King and three prisoners King said: Everybody is wearing a hat, a hat is either black or white. The prisoners can see each other and can see other people's hat color, but can not see the color of his own hat. No speaking is allowed. King actually gave them wearing a black hat. And set the following rules: 1 The prisoners who saw two other people are white hat, can be released. (2) knows his black hat, can be released. After some time, A prisoner finds his hat is black and was released, how did he do so?