Count Total Set Bits

//User function Template for C++

class Solution{
    public:
    int largestPower2(int n){
        int x = 0;
        while((1<<x) <= n){
            x++;
        }
        return x-1;
    }
    
    int countSetBits(int n)
    {
        if(n == 0) return 0;
        int x = largestPower2(n);
        
        int uptoPowerX = (1<<(x-1))*x;
        
        int uptoNFromX = n-(1<<x)+1;
        
        int rest = countSetBits(n-(1<<x));
        
        return uptoPowerX + uptoNFromX + rest;
    }
};