1. (8 points) The sum of the digits of the result of the expression $999999999-88888888+7777777-666666+55555-4444+333-22+1$ is $\qquad$ .
【Solution】Solution: Since there is no carry or borrow in the calculation process, the sum of the digits of the result is the sum of the digits of each number in the formula added and subtracted,

Original expression $=9 \times 9-8 \times 8+7 \times 7-6 \times 6+5 \times 5-4 \times 4+3 \times 3-2 \times 2+1 \times 1(\bmod 10)$
$$
=(9+8)(9-8)+(7+6)(7-6)+\cdots+(3+2)(3-2)+1=9+8+7+6+5+4+3+2+1=45 \text {, }
$$

Therefore, the answer is 45.