You will be surprised.

I use do...while instead of the for loop, making the usual nasty C-programmer use of ++ in the right spots, but other than that, my solution is the same.

Edit: Correction... I also copy the '\0' character before exiting instead of making that a separate step. It has to do with the order of things. Here is the code:

   char* read_ptr = s;
   do {
      if (*read_ptr != c) {
         *s++ = *read_ptr;
      }
   } while (*read_ptr++ != '\0');

You can use an inner loop to skip over removable chars in the source string but I didn't think that was n^2. (Code below for reference.) Am I wrong that my solution is n, or is there another obvious n^2 algorithm?

  void condense_by_removing( char *z_terminated , char char_to_remove)
  {
    char * sp;
    char * dp;
    sp = dp = z_terminated;
    while (*sp != '\0') {
      while (*sp == char_to_remove){
        sp++;
      }
      *dp = *sp;
      dp++;
      sp++;
    } 
    *dp = *sp;
  }