Merkkijono

#include <iostream>
#include <vector>
#include <set>
#include <map>

using namespace std;

int main()
{
	std::ios_base::sync_with_stdio(0);
	std::cin.tie(0);
	int n;
	std::cin >> n;
	if (n == 1)
	{
		printf("a");
		return 0;
	}

	map<char, string> m;
	const char* a = "abcdefghijklmnopqrstuvwxyz";
	string res = "";
	for (int i = n-2; i >= 0; i--) 
	{
		string s = "";
		char letter = a[i];
		char padc = a[i+1];
		string pad = m.count(padc) > 0 ? m.at(padc) : string(1, padc);
		s += pad;
		s += letter;
		s += pad;
		m[letter] = s;
		if (i==0) printf("%s", s.c_str());
	}
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED