I think the expression of LZ is not very clear. It should be: when playing cards, deal cards one by one, in 7 rows, with 3 cards in each row (instead of 7 cards in 3 rows). When collecting cards, collect cards in column order, the column that receives cards is folded up, and the other two columns are folded up and placed above it. Then repeat the above card release and card collection.

1, because when the card is received, the column with that card is folded, and the other two columns are folded and placed above and below it. Therefore, when the card is received for the first time, it becomes at least the eighth card in the whole stack (whether it is a positive card or a negative card, the same below), because there are seven more cards before and after it.

2. Because this card has become at least the eighth card in the whole stack, the card in front of it can be dealt for at least 2 rows at the second time. Therefore, at least it becomes the eighth card in the whole stack +2 = 10 when collecting cards.

3. Because this card has at least become the 10 card of the whole deck, the cards in front of it can be dealt for at least 3 rows in the third deal. Therefore, at least it becomes the eighth card in the whole stack +3 = 1 1.

As mentioned above, since both positive and negative numbers meet the above conditions, after dealing and receiving cards for three times, the card must be 1 1 (whether positive or negative), that is, the card in the middle of the whole stack (if the positive number is 12, it will be 10 in turn.

I said you should understand, right?