C++: Find 1st non-repeating character

Here is a function that I enjoyed solving. You have to write a function that takes a string input and finds the first non-repeating character in a string (eg. “ABCA” -> B). This code runs in about O(n). Enjoy!

/*FUNCTION*/ //*FIND 1ST NON-REPEATING****************************************
char find_nonrepeat_char(strLetters) {
//make ascii array to store letter count, then count
{
  int countArray[25]; 
  for (int i =0; i < 26; i++) countArray[i] = 0;
  for (int i = 0; i < length(strLetters); i++) {  
    countArray[(int)strLetters[i]-65]++;           
  } 
  for (i = 0; i < length(strLetters); i++) {      
    if (countArray[(int)strLetters[i]-65)] == 1) return strLetters[i];
  }
}