6. (5 points) There is a class of four-digit numbers, where the sum of any two adjacent digits is no greater than 2. If these numbers are arranged in ascending order, what is the second to last number? $\qquad$ .
【Answer】Solution: Largest number: thousand's place maximum 2, hundred's place 0, ten's place maximum 2, unit's place 0, 2020,
Second: thousand's place maximum 2, hundred's place 0, ten's place maximum 1, unit's place 1, 2011,
Third: thousand's place maximum 2, hundred's place 0, ten's place maximum 1, unit's place 0, 2010, ..

If arranged from smallest to largest, the second to last number is 2011, so the answer is: 2011.