Merkkijono

input/code.cpp: In function 'int main()':
input/code.cpp:26:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int j = 0; j < str.length()-added; j++)
                   ~~^~~~~~~~~~~~~~~~~~~~

#include <iostream>
#include <string>
#include <math.h>
#include <map>
#include <algorithm>
#include <vector>
#include <iostream>
#include <fstream>
#include <set>
#include <unordered_map>

using namespace std;
typedef unsigned long long ll;
//https://cses.fi/dt/

int main()
{
	string str = "a";
	int n;
	cin >> n;
	char c = 'a';
	for (int i = 1; i < n; i++)
	{
		c++;
		int added = 0;
		for (int j = 0; j < str.length()-added; j++)
		{
			str.insert(j+added, string(1,c));
			added++;
		}
		str.append(string(1,c));
	}
	cout << 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