Number of 1 Bits - Implementation

Here is the implementation:

function hammingWeight(n):
    count = 0
    while n != 0:
        n = n & (n - 1)
        count += 1
    return count

This runs in $O(k)$ time where $k$ is the number of set bits. Worst case is $O(\log n)$ for 32/64 bit integers.