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This problem can be solved by finding recurrence relation using exponential generating functions. Becuase we calculate mod 10^9 + 7 we must find modulo inverses for factorials. The recurrence relation can also be found using pure combinatorics.
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Project Euler #172: Investigating numbers with few repeated digits
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This problem can be solved by finding recurrence relation using exponential generating functions. Becuase we calculate mod 10^9 + 7 we must find modulo inverses for factorials. The recurrence relation can also be found using pure combinatorics.