Search Pattern (Rabin-Karp Algorithm)

Solution
{
    int mul=256,mod=107;
    public:
        vector <int> search(string pat, string txt)
        {
            int lenTxt=txt.length(),lenPat=pat.length();
            if(lenTxt<lenPat)return {-1};
            vector<int>pos;

            int patHash=0,txtHash=0,hash=1;

            for(int i=1;i<=lenPat-1;i++){
                hash=(hash*mul)%mod;
            }

            for(int i=0;i<lenPat;i++){
                patHash=(patHash*mul + pat[i])%mod;
                txtHash=(txtHash*mul + txt[i])%mod;
            }

            for(int i=0;i<=lenTxt-lenPat;i++){

                if(patHash==txtHash){
                    int present=true;
                    for(int j=0;j<lenPat;j++){
                        if(txt[i+j] != pat[j]){
                            present=false;
                            break;
                        }
                    }
                    if(present==true)pos.push_back(i+1);
                }

                if(i<lenTxt-lenPat){
                    txtHash=((txtHash- (txt[i]*hash)%mod )*mul + txt[i+lenPat])%mod;
                    if(txtHash<0)txtHash+=mod;
                }
            }
            if(pos.size()==0)return {-1};
            return pos;
        }

};