Leetcode 191: number of 1 bits

In this Leetcode question, we are asked to count and return the number of 1s in the bit representation of 32-bit integers.

Problem statement

Essentially, the problem is quite simple to solve, we can look bit by bit by checking (n & 1) and divide n by 2, with bitwise op n >>= 1;.

However, it get slightly more complicated with negative numbers and the 2-complement representation. In the 2 complement binary representation, if we take a positive integer n, flipping every single bit will result in -(n+1). The reason it is not -n is that however it would be simpler, flipping all the bits for 0 would result in 0 and that is not acceptable to have 2 different representations of the same number for practical purpose.

To solve the problem in the negative case. I flipped n to its positive representation and counted the inverse, that is I started with 32 1s and each time I encounter a 1 in the complement representation I remove 1 from the final result.

Here is the full solution:

public class Solution {
    // you need to treat n as an unsigned value
    public int hammingWeight(int n) {
        int res = 0;
        if (n < 0) {
            n = - n - 1;
            res = 32;
            while (n != 0) {
                if ((n & 1) != 0) {
                    res--;
                }
                n >>= 1…