Explain

class Solution {
public:
    int hammingWeight(uint32_t n) {
        int result = 0;
        while(n)
        {
            n &= (n-1);
            result++;
        }
        return result;
    }
};

n & (n - 1) drops the lowest set bit. It’s a neat little bit trick.

Let’s use n = 00101100 as an example. This binary representation has three 1s.

If n = 00101100, then n - 1 = 00101011, so n & (n - 1) = 00101100 & 00101011 = 00101000. Count = 1.

If n = 00101000, then n - 1 = 00100111, so n & (n - 1) = 00101000 & 00100111 = 00100000. Count = 2.

If n = 00100000, then n - 1 = 00011111, so n & (n - 1) = 00100000 & 00011111 = 00000000. Count = 3.

n is now zero, so the while loop ends, and the final count (the numbers of set bits) is returned.

using STL

class Solution {
public:
    int hammingWeight(uint32_t n) {
        return bitset<32>(n).count();
    }
};