Find the first un-repeated character in a string

Question

What is the quickest way to find the first character which only appears once in a string?

Answer 1

in C, this is almost Shlemiel the Painter's Algorithm (not quite O(n!) but more than 0(n2)).

But will outperform "better" algorithms for reasonably sized strings because O is so small. This can also easily tell you the location of the first non-repeating string.

char FirstNonRepeatedChar(char * psz)
{
   for (int ii = 0; psz[ii] != 0; ++ii)
   {
      for (int jj = ii+1; ; ++jj)
      {
         // if we hit the end of string, then we found a non-repeat character.
         //
         if (psz[jj] == 0)
            return psz[ii]; // this character doesn't repeat

         // if we found a repeat character, we can stop looking.
         //
         if (psz[ii] == psz[jj])
            break; 
      }
   }

   return 0; // there were no non-repeating characters.
}

edit: this code is assuming you don't mean consecutive repeating characters.