1. Number of 1 Bits

public class Solution {
    // you need to treat n as an unsigned value
    public int hammingWeight(int n) {
        int count = 0, mask = 1;
        for(int i = 0; i < 32; i++){
            if((n & mask) != 0) count++;
            mask = mask << 1;
        }
        return count;
    }
}

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

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