#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include <stdbool.h>
#include <assert.h>

char *getSpecialString(
    char *buffer,
    int bufferLength,
    const char *str,
    const char *str2);

int main()
{
    int bufLen = 2;
    char buf[bufLen];
    char *result = getSpecialString(
        buf,
        bufLen,
        "Hhel12323Llmo th34-ere!!",
        "hll e!");
    assert(0 == strcmp(result, "h"));
    return EXIT_SUCCESS;
}

char *getSpecialString(
    char *buffer,
    int bufferLength,
    const char *str,
    const char *str2)
{
    int numberOfLetters = 'z' - 'a' + 1;
    bool letterIsPresent[numberOfLetters];
    for (int i = 0; i < numberOfLetters; i++)
    {
        letterIsPresent[i] = false;
    }
    int str2Len = strlen(str2);
    for (int i = 0; i < str2Len; i++)
    {
        if (islower(str2[i]))
        {
            int n = str2[i] - 'a';
            letterIsPresent[n] = true;
            if (n >= 1)
            {
                letterIsPresent[n - 1] = true;
            }
            if (n < numberOfLetters - 1)
            {
                letterIsPresent[n + 1] = true;
            }
        }
    }
    int index = 0;
    int strLen = strlen(str);
    for (int i = 0; index < bufferLength - 1 && i < strLen; i++)
    {
        if (islower(str[i]) && letterIsPresent[str[i] - 'a'])
        {
            buffer[index] = str[i];
            index++;
        }
    }
    buffer[index] = '\0';

    return buffer;
}