C++ :: Updating String From Aaaa To Dddd

I am writing some code that takes a string of n letters, so a, or aa or aaa or aaaa...a and transforms these into d or dd or ddd or dddd...d by essentially counting. e.g aaaa -> aaab -> aaac -> aaad -> aaba and so on. Also technically the user inputs what the ending is whether it be d or e,f,g; however, for practical reasons counting up to dddd should work. My main issue is I am having trouble getting the letters correct. I get up to aadd and then once I go to abaa I have trouble getting the second to last a to switch correctly.

Code:
#include<iostream>
#include<string>
#include<vector>
using namespace std;
int main() {
vector<string> pleasework;
string apple = "aaaaaa";
int end = apple.size()-1;

[Code] ....