Number of 1 Bits

EasyBits and mathAlso known as LeetCode 191

You get a non-negative integer. Return the number of bits that are 1 in its binary representation. This count is sometimes called the Hamming weight.

For example 11 is 1011 in binary, which has three 1 bits.

Input n = 11

Output 3

11 is 1011 in binary.

Input n = 128

Output 1

128 is 10000000 in binary.

Input n = 2147483645

Output 30

Only one bit of the 31 low bits is 0.

Common mistakes

Using a right shift on a negative number in a language that keeps the sign bit, so n never reaches 0 and the loop never ends.
Counting digits of the decimal number instead of bits of the binary number.
Writing while (n > 0) in a language where n could be negative, which stops at once. Here n is never negative, but know why the loop is safe.

Questions an interviewer may ask next

Count the 1 bits for every number from 0 to n. (Counting Bits: reuse earlier answers.)
The function will be called a million times on 32 bit values. How would you speed it up? (A lookup table of 256 entries, or the hardware popcount instruction.)
Allow negative numbers as 32 bit two's complement. What changes in a language where a right shift keeps the sign?
Find the number of bit positions where two numbers differ. (XOR them, then count the 1 bits.)

Expected output

Run checks these cases. Submit also runs hidden cases, and shows you the first one that fails.