#include <iostream>
#include <string>

using namespace std;

void MergeSort(string & s);

int main() {
    string phrase;

    cout << "Enter a string " ;
    getline (cin, phrase);
    cout << endl;

    cout << "Sorting \"" << phrase << '"' << endl;

    MergeSort(phrase);

    cout << "That becomes \"" << phrase << '"' << endl;

    return 0;
}

void MergeSort(string & s) {
    size_t pos;

    // substrings
    string a,b;

    // for merging
    size_t aPos = 0, 
           bPos = 0;

    // base case, nothing to sort.
    if (s.size() < 2) {
       return;
    }

    //split into two substrings.
    pos = s.size()/2;
    a = s.substr(0,pos);
    b = s.substr(pos, string::npos);

    // sort the two substrings
    MergeSort(a);
    MergeSort(b);

    // merge the results.
    // initialize the return string to empty.
    s = "";

    // merge the two lists.
    while (aPos < a.size() and bPos < b.size() ) {
        if (a[aPos] < b[bPos]) {
	   s += a[aPos];
	   aPos++;
	} else {
	   s += b[bPos];
	   bPos++;
	}
    }

    // add the part left over in the remaining substring 
    if (aPos < a.size()) {
        s += a.substr(aPos, string::npos);
    } else {
        s += b.substr(bPos, string::npos);
    }

    return;
}