Bit trick to detect powers of two

How can you check quickly whether a given integer is a power of two?

Test n > 0 && (n & (n - 1)) == 0. A power of two has exactly one bit set, and subtracting $1$ flips that bit and turns on all lower bits, so the AND is zero; any other positive integer keeps its highest set bit. This is $O(1)$; the check $n>0$ excludes zero and negatives.