Merkkijono

input/code.cpp: In function 'std::__cxx11::string add_padding(std::__cxx11::string, char)':
input/code.cpp:11:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < s.length(); ++i)
                 ~~^~~~~~~~~~~~

#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
using namespace std;

string add_padding(string s, char c)
{
	string ss = "";
	ss.push_back(c);
	for(int i = 0; i < s.length(); ++i)
	{
		ss.push_back(s[i]);
		ss.push_back(c);
	}
	return ss;
}

int main()
{
	int n;
	cin >> n;

	string s = "a";
	for(int i = 1; i < n; ++i)
	{
		s = add_padding(s, (char)'a'+i);
	}
	cout << s << "\n";
	return 0;
}

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