Number of 1 Bits

How you get there

1. 1n & (n - 1) clears the lowest set bit.
2. 2Repeat until zero and count the iterations.
3. 3That runs once per set bit rather than once per bit position.

Solution

def hamming_weight(n: int) -> int:
    count = 0
    while n:
        n &= n - 1        # clears the lowest set bit
        count += 1
    return count

Time

O(number of set bits)

Space

O(1)

What goes wrong

Shifting through all 32 positions works but is slower on sparse inputs.