Number of 1 Bits: the idea to remember.

The core insight

Repeatedly replace n with n AND n minus one; each replacement clears exactly one set bit.

Why it works

Subtracting one flips the lowest one to zero and the lower zeros to ones. AND therefore removes only that one.

The mistake to avoid

Use n != 0, not n > 0, if interpreting signed Java ints as all 32 bits.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

int hammingWeight(int n) {
    int count = 0;
    while (n != 0) {
        n &= n - 1;
        count++;
    }
    return count;
}

Make the next connection