Merkkijono

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#pragma GCC diagnostic ignored "-Wunused-result"

int main()
{
    unsigned long n;

    scanf("%lu", &n);

    unsigned long length = 1;

    for(unsigned long i = 1; i < n; i++) { length = length * 2 + 1; }

    char* s = (char*)alloca(length + 1);

    s[length] = 0;
    s[0] = 'a';

    unsigned long index = 1;

    for(unsigned long i = 1; i < n; i++)
    {
        s[index] = 'a';
        for(unsigned long j = 0; j < index; j++) { s[j]++; }
        memcpy(s + index + 1, s, index);

        index = index * 2 + 1;
    }

    printf("%s", s);
}

#pragma GCC diagnostic pop

//dcdbdcdadcdbdcd
//dcdbdcdadcdbdcd

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