I've found out why I did wrong in that logic problem.

     Few days ago, I wrote down something about a logic problem, in which I encounter a problem that I couldn't understand. Today, when I was having lunch, I spended a few minutes in thinking the problem. And I've understood it now. I answered the problem like this: "In the list, the 49 statements in the front is true, while others are false.", but the key is "This cannot happen; it is a paradox, showing that these cannot be statements". To understand this, we should focus on the statements right after the 49th, namely, the 50th. What's the truth value of it? If it is true, then according to its meaning, there will be at most 49 true statement in the list. But according the conclusion I wrote before, if the 50th statement is ture, then the 50 statements in the front of the list will consequently be true. Obviously, 49 doesn't equals 50. So there is a contradiction here, making the list a paradox.