191. Number of 1 Bits

Write a function that takes an unsigned integer and returns the number of ’1' bits it has (also known as the Hamming weight).

For example, the 32-bit integer ’11' has binary representation00000000000000000000000000001011, so the function should return 3.

Solution 1: using a for loop to clear one bit set a time.

    // you need to treat n as an unsigned value
    public int hammingWeight(int n) {
        int res = 0;
        while (n != 0) {
            n = n&(n-1);
            res++;
        }
        return res;
    }

Solution 2: using cardinality() in Java's BitSet class. (n may out of range)

    // you need to treat n as an unsigned value
    public int hammingWeight(int n) {
        return BitSet.valueOf(new long[]{n}).cardinality();
    }