Halfabet code

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>    // std::remove_if
#include <fstream>
using namespace std;
bool checkExists(vector<string> v, string key)
{
    bool returnString=false;

    for (int i=0; i<v.size(); i++)
    {
        if (v[i]==key)
            returnString=true;
    }
    return returnString;

}
void cleanup(string &s)
{
    s.erase(remove_if(s.begin(), s.end(), ::ispunct), s.end());
}
string downSize(string &line)
{
    char x;
    for(char& c : line)
    {
        x = c;
        if (c > 64 && c < 90)
        {
            c += 32;
        }
        replace(line.begin(), line.end(), x, c);
    }
    return line;
}

bool isValid(string s)
{
    for (int i = 0, j = 1; j < s.length(); i++, j++)
    {
        if (toupper(s[i]) >= 'A' && toupper(s[i]) < 'N')
        {
            if (toupper(s[j]) < 'N')
                return false;
        }
        else if (toupper(s[i]) >= 'N')
        {
            if (toupper(s[j]) >= 'N')
                return false;
        }
    }

    return true;
}

int whichList(string halfabet)
{
    int returnInt = -1;

    //if starts with a-m and ends with a-m
    if (halfabet[0]>= 'a' && halfabet[0] <'n')
        if (halfabet[halfabet.length()-1]>= 'a' && halfabet[halfabet.length()-1] < 'n')
            returnInt=0;
    //if starts with a-m and ends with m-z
    if (halfabet[0]>= 'a' && halfabet[0] <'n')
        if (halfabet[halfabet.length()-1] >= 'n')
            returnInt=1;

    //if starts with a-m and ends with m-z
    if (halfabet[0]>= 'n')
        if (halfabet[halfabet.length()-1]>= 'a' && halfabet[halfabet.length()-1] < 'n')
            returnInt=2;

    //if starts with m-z and ends with m-z
    if (halfabet[0] >= 'n')
        if (halfabet[halfabet.length()-1] >= 'n')
            returnInt=3;

    //return -
    //0 = FF
    //1 = FL
    //2 = LF
    //3 = LL
    return returnInt;
}



int main()
{
    vector<string> FF;
    vector<string> FL;
    vector<string> LF;
    vector<string> LL;

    ifstream fin;
    fin.open("Book.txt");
    string word;
    //if file is open
    if(fin.is_open())
    {
        //while file has text
        while (!fin.eof())
        {
     fin>>word;
        downSize(word);
      //clean it up for punctuation
       cleanup(word);
        bool halfabetCheck = isValid(word);
        if (halfabetCheck)
        {
        //find list
            //if 0..1..2...3 push back etc
            int list =  whichList(word);

            if (list == 0)
            {
                //check if exists already
                if (!checkExists(FF,word))
                {
                    FF.push_back(word);
                }
            }


            if (list == 1)
            {
                //check if exists already
                if (!checkExists(FL,word))
                {
                    FL.push_back(word);
                }

            }


            if (list == 2)
            {
                //check if exists already
                if (!checkExists(LF,word))
                {
                    LF.push_back(word);
                }
            }


            if (list == 3)
            {
                //check if exists already
                if (!checkExists(LL,word))
                {
                    LL.push_back(word);
                }
            }
        }
            ofstream foutFF;
            foutFF.open("FF.txt");
            ofstream foutFL;
            foutFL.open("FL.txt");
            ofstream foutLF;
            foutLF.open("LF.txt");
            ofstream foutLL;
            foutLL.open("LL.txt");

            for (int i=0; i<FF.size(); i++)
            {

                foutFF<<FF[i]<<endl;
            }

            for (int i=0; i<FL.size(); i++)
            {

                foutFL<<FL[i]<<endl;
            }
            for (int i=0; i<LF.size(); i++)
            {

                foutLF<<LF[i]<<endl;
            }
            for (int i=0; i<LL.size(); i++)
            {

                foutLL<<LL[i]<<endl;
            }

        }


    }
    fin.close();

    return 0;
}