I exist sometimes

Given a list of integers \(1\cdots n\), return the count of the number that have the number \(k\) in them. For \(n\leq10^{9}\) how many numbers have the number \(7\) in them.

Details and Assumptions

For \(n\leq10\), the number of occurrence of \(7\) is one.

For \(n\leq100\) the number of occurrence of \(7\) is \(19\)...\(7,17,27,37,47,57,67,70,71\cdots ,79,87,97\)

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