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

int main()
{
    char symbols[16] = "";
    char symbols_copy[16];
    int index = 0;
    while (index++ < 15)
    {
         memcpy(symbols_copy, symbols, sizeof(symbols));

         if (index % 2)
             snprintf(symbols, sizeof symbols, "a%s", symbols_copy);
         else
             snprintf(symbols, sizeof symbols, "b%s", symbols_copy);

         printf("%s\n", symbols);
    }
    return 0;
}