first_non_repeating_character.c

// C program to find first non-repeating character
#include<stdlib.h>
#include<stdio.h>
#define NO_OF_CHARS 256

/* Returns an array of size 256 containg count
   of characters in the passed char array */
int *getCharCountArray(char *str)
{
   int *count = (int *)calloc(sizeof(int), NO_OF_CHARS);
   int i;
   for (i = 0; *(str+i);  i++)
      count[*(str+i)]++;
   return count;
}

/* The function returns index of first non-repeating
   character in a string. If all characters are repeating
   then returns -1 */
int firstNonRepeating(char *str)
{
  int *count = getCharCountArray(str);
  int index = -1, i;

  for (i = 0; *(str+i);  i++)
  {
    if (count[*(str+i)] == 1)
    {
      index = i;
      break;
    }
  }

  free(count); // To avoid memory leak
  return index;
}

/* Driver program to test above function */
int main()
{
  char str[] = "example to check";
  int index =  firstNonRepeating(str);
  if (index == -1)
    printf("Either all characters are repeating or string is empty");
  else
   printf("First non-repeating character is %c", str[index]);
  getchar();
  return 0;
}