Multiple of 9

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You are given an $N$-digit integer with some unknown digits (marked with $?$). Count the number of ways to fill in each $?$ with a digit ($0$ to $9$) such that the resulting integer is **positive** and a multiple of $9$. The integer cannot have leading zeroes. Two ways are different if, for some $?$, the chosen digit is different. Even though the number of ways can be very large, you should o

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solution.cppC++17

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