Count Bits - Implementation

Here is the complete solution using the Brian Kernighan trick:

function hammingWeight(n)
    count := 0
    while n > 0
        n := n & (n - 1)    // Flip rightmost 1 bit to 0
        count := count + 1
    return count

For $n = 11$ (binary $1011$), the loop runs 3 times: $1011 \& 1010 = 1010$, $1010 \& 1001 = 1000$, $1000 \& 0111 = 0000$. The count is 3. Time complexity: $O(k)$ where $k$ is the number of 1 bits. Space: $O(1)$.