I am trying to see if every letter of the alphabet is used exactly once in a key to encrypt each letter of the alphabet. It would then store everything in a list, and once the list hits 26, exit the loop. I am having a bit of an issue achieving this (new to coding)

//check if all letters of the alphabet are in the key.  Check if araay is in range a-z (already lowercase), if so, add it to list.  return 1 if it is in the list.
//for each character in key:
// Create an array to store the letters of the key
    if (c < 'a' || c > 'z') {
        printf("String contains characters other than 'a' to 'z'.n");
        return 0;
    }

    if (letters[c - 'a'] == 1) {
        printf("String contains duplicate characters.n");
        return 0;
    }

    letters[c - 'a'] = 1;

I have tried a few videos and articles but struggling to understand it if anyone can explain it /point me to good resources for it.