如何检查一个整数是否是 3 的幂? [英] How to check if an integer is a power of 3?
问题描述
我看到了 来计算常数.例如对于 2^63-1
(signed int64),C++ 和 Java 都给出 4052555153018976256
,但正确的值是 4052555153018976267
.
I saw this question, and pop up this idea.
There exists a constant time (pretty fast) method for integers of limited size (e.g. 32-bit integers).
Note that for an integer N
that is a power of 3 the following is true:
- For any
M <= N
that is a power of 3,M
dividesN
. - For any
M <= N
that is not a power 3,M
does not divideN
.
The biggest power of 3 that fits into 32 bits is 3486784401
(3^20
). This gives the following code:
bool isPower3(std::uint32_t value) {
return value != 0 && 3486784401u % value == 0;
}
Similarly for signed 32 bits it is 1162261467
(3^19
):
bool isPower3(std::int32_t value) {
return value > 0 && 1162261467 % value == 0;
}
In general the magic number is:
== pow(3, floor(log(MAX) / log(3)))
Careful with floating point rounding errors, use a math calculator like Wolfram Alpha to calculate the constant. For example for 2^63-1
(signed int64) both C++ and Java give 4052555153018976256
, but the correct value is 4052555153018976267
.
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