Riddle of the day

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Each day, I'll post a riddle and you can try to solve it. The first riddle: A prison warden wants to release one of his brighter prisoners. He gets 4 of his smart prisoners and tells them the following: "I have 4 hats here, 2 white and 2 black. Each of you will have a hat put on your head". he then further explains the 3 of them will be lined up and a fourth will be in a different room. The prisoners then each get a hat (no can see his own hat), the fourth prisoner is led to a very far, soundtight room. The other 3 are put in a line in front of the wall. Of the three, the first prisoner sees nothing but the wall, the second sees the first prisoner (and his hat color), the third one sees the first 2 prisoners and their two hats. He tells them that the first prisoner to shout out his hat color, will be freed. if he makes a mistake, he'll be put to death. After several minutes, a prisoner shouts out the color of his hat correctly and is freed, which prisoner was it and how did he know ?