First of all, we must find a breakthrough to solve the problem. Only one of the four people told the truth. Analyzing the confessions of four people, we can find that what they said is contradictory. B said Ding was a criminal and Ding said I was not a criminal. What they said must be true or false. I also know that only one of the four people is telling the truth, so what A and B say is a lie. A says I am not a criminal, so in fact A is a criminal.