How to check if a number is a power of 5

Question

How to check if a number is power 5?

I could think of a downstream algorithm. Is there any way to improve it? Any math trick?

• First check if the last digit has 5.
• If the last digit is 5; divide it by 5.
  If the result of the division is 1, then the number is 5. Once again, check whether the result of the division is the force 5 itself (that is, go to step 1 with the result as a number).

Answer 1

You do not need to look at individual numbers, you can just do it like this:

n = (int)(log(x) / log(5)); // get n = log5(x), truncated to integer
if (pow(5, n) == x)         // test to see whether x == 5^n
    // x is a power of 5

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Answer 2

int/long range , ( 60 ), HashSet, O (1)

Answer 3

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Answer 4

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