The problem link is Power Of Four.
Given an integer (signed 32 bits), write a function to check whether it is
a power of 4.
If a is power of 4, a or 0x55555554 equals 0x5555554.But not vice versa.So I first test if a is power of 2,means a only one 1 bit.
public static boolean isPowerOfFour(int num) {
return num>=4 && ((num & (num - 1)) == 0) &&
((num | 0x55555554) == 0x55555554);
}