请教一道算法题目

Design an algorithm that uses these two digital counters
jointly to handle the sequence of n operations adding or subtracting digits, i.e., where a1; a2; a3; : : <-D {0,1,2,...,9}, so that the total cost is O(n)

因为进位有cost，所以怎么可以让O（n）时间内加法和减法实现，使用两个计数器。