BigInt Multiplication

string multiply(string a,int b) {
        // a contains the biginteger in reversed form
    int carry=0;
    for(int i=0;i<a.size();i++) {
        carry+=(a[i]-48)*b;
        a[i]=(carry%10+48);
        carry/=10;
    }
    while(carry) {
        a+=(carry%10+48);
        carry/=10;
    }
    // a contains the biginteger in reversed form
    return a;
}