Getting an error : Unhandled exception thrown: write access violation

char* reverse(char* mystring) //revere the string 
{
  int len = LenString(mystring);
  int i, j;
  // char *str = new char;
  char *str = mystring;
  i = 0;
  j = len - 1;
  str = mystring;
  char temp;
  char temp1;
  while (i < j)
  {
    temp = str[i];
    temp1 = str[j];
    str[i] = temp1;
    str[j] = temp;
    i++;
    j--;
  }

  return str;
}

I tried to dynamically allocate array as well but still getting same error. If I need to make some changes do let me know. I cannot change my function from char* reverse(char* mystring) to any other type.