Count Total Set Bits

• Time : O(logA)
• Space : O(1)

c++

int Solution::solve(int A) {
    if(A <= 1) return A;
    long long lg2 = log2(A), mask = 1<<lg2, mod = 1e9 + 7;
    long long res = 0, shift = 0;
    
    while((1<<shift) < mask) res = (res * 2 + (1<<shift++)) % mod;
    
    return (res + ((A^mask) + 1) + solve(A ^ mask)) % mod;
}