Checking if a string is an isogram

A string is an isogram if no character in it appears more than once.

Let us write a simple algorithm to identify if a given input string is an isogram. For simplicity let us consider only strings that have small characters and no spaces or special characters i.e. - only characters from a to z.

In our previous post, we learnt how to keep track of characters that appear in a string as we traverse the input string. We use the same idea here, just this time we simplify it to be 26 characters (a to z) instead of 256.

We traverse the input string and maintain a counter for each of the occurences in the string. Note that we subtract ‘a’ from the ASCII value as our strings contain only a to z and we want our index 0 to correspond to a. The intention of this simplification was precisely to learn how to modify our algorithm for such specific use cases.
We traverse the counter array (which is essentially a mapping seen: character -> count to see if any of the characters appeared more than once (i.e. their count mapping is greater than 1).

#define MAX_CHARS 26
bool isogram(std::string s)
{
  int seen[MAX_CHARS] = {0};
  int slen = s.length();
  bool res = true;

  // (1)
  for (int i = 0; i < slen; i++)
    seen[s[i] - 'a']++;

  // (2)
  for (int i = 0; i < 26; i++)
  {
    if (seen[i] > 1)
    {
      res = false;
      break;
    }
  }
  return res;
}