http://nabetani.sakura.ne.jp/hena/ord4tetroid/

tetroid.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

const struct {
  int t[3];
  char c;
} lut[] = {
  {{ 1, 10, 11}, 'O'},
  {{ 1,  2,  3}, 'I'},
  {{10, 20, 30}, 'I'},
  {{ 1,  2, 11}, 'T'},
  {{ 9, 10, 11}, 'T'},
  {{ 9, 10, 20}, 'T'},
  {{10, 11, 20}, 'T'},
  {{ 1,  9, 10}, 'S'},
  {{ 1, 11, 12}, 'S'},
  {{ 9, 10, 19}, 'S'},
  {{10, 11, 21}, 'S'},
  {{ 1,  2, 12}, 'L'},
  {{ 1,  2, 10}, 'L'},
  {{ 1, 10, 20}, 'L'},
  {{ 1, 11, 21}, 'L'},
  {{ 8,  9, 10}, 'L'},
  {{10, 11, 12}, 'L'},
  {{10, 19, 20}, 'L'},
  {{10, 20, 21}, 'L'},
};
const int lut_size = sizeof(lut) / sizeof(lut[0]);

int cmp_int(const void *a, const void *b)
{
  return *(const int*)a - *(const int*)b;
}

char solve(const char *s)
{
  int b[4];
  int i;

  sscanf(s, "%d,%d,%d,%d", b, b+1, b+2, b+3);
  qsort(b, 4, sizeof(int), cmp_int);
  for (i = 3; 0 <= i; i--)
    b[i] -= b[0];
  for (i = 0; i < lut_size; i++) {
    if (memcmp(b+1, lut[i].t, sizeof(int)*3) == 0)
      return lut[i].c;
  }
  return '-';
}

void test(const char *s, char expect)
{
  char result = solve(s);
  if (result != expect)
    printf("%s:%c!=%c\n", s, result, expect);
}

int main()
{
  /*#1*/ test("55,55,55,55", '-');
  /*#2*/ test("07,17,06,05", 'L');
  /*#3*/ test("21,41,31,40", 'L');
  /*#4*/ test("62,74,73,72", 'L');
  /*#5*/ test("84,94,74,75", 'L');
  /*#6*/ test("48,49,57,47", 'L');
  /*#7*/ test("69,89,79,68", 'L');
  /*#8*/ test("90,82,91,92", 'L');
  /*#9*/ test("13,23,03,24", 'L');
  /*#10*/ test("24,22,25,23", 'I');
  /*#11*/ test("51,41,21,31", 'I');
  /*#12*/ test("64,63,62,65", 'I');
  /*#13*/ test("49,69,59,79", 'I');
  /*#14*/ test("12,10,21,11", 'T');
  /*#15*/ test("89,99,79,88", 'T');
  /*#16*/ test("32,41,43,42", 'T');
  /*#17*/ test("27,16,36,26", 'T');
  /*#18*/ test("68,57,58,67", 'O');
  /*#19*/ test("72,62,61,71", 'O');
  /*#20*/ test("25,24,15,14", 'O');
  /*#21*/ test("43,54,53,42", 'S');
  /*#22*/ test("95,86,76,85", 'S');
  /*#23*/ test("72,73,84,83", 'S');
  /*#24*/ test("42,33,32,23", 'S');
  /*#25*/ test("66,57,67,58", 'S');
  /*#26*/ test("63,73,52,62", 'S');
  /*#27*/ test("76,68,77,67", 'S');
  /*#28*/ test("12,11,22,01", 'S');
  /*#29*/ test("05,26,06,25", '-');
  /*#30*/ test("03,11,13,01", '-');
  /*#31*/ test("11,20,00,21", '-');
  /*#32*/ test("84,95,94,86", '-');
  /*#33*/ test("36,56,45,35", '-');
  /*#34*/ test("41,33,32,43", '-');
  /*#35*/ test("75,94,84,95", '-');
  /*#36*/ test("27,39,28,37", '-');
  /*#37*/ test("45,34,54,35", '-');
  /*#38*/ test("24,36,35,26", '-');
  /*#39*/ test("27,27,27,27", '-');
  /*#40*/ test("55,44,44,45", '-');
  /*#41*/ test("70,73,71,71", '-');
  /*#42*/ test("67,37,47,47", '-');
  /*#43*/ test("43,45,41,42", '-');
  /*#44*/ test("87,57,97,67", '-');
  /*#45*/ test("49,45,46,48", '-');
  /*#46*/ test("63,63,52,72", '-');
  /*#47*/ test("84,86,84,95", '-');
  /*#48*/ test("61,60,62,73", '-');
  /*#49*/ test("59,79,69,48", '-');
  /*#50*/ test("55,57,77,75", '-');
}

tetroid2.c

const struct {
  int d[6];
  char c;
} lut[] = {
  {{1,1,1,2,4,5}, 'L'},
  {{1,1,1,4,4,9}, 'I'},
  {{1,1,1,2,2,4}, 'T'},
  {{1,1,1,1,2,2}, 'O'},
  {{1,1,1,2,2,5}, 'S'},
};
const int lut_size = sizeof(lut) / sizeof(lut[0]);

int cmp_int(const void *a, const void *b)
{
  return *(const int*)a - *(const int*)b;
}

int dist(int a, int b)
{
  return (a/10-b/10)*(a/10-b/10) + (a%10-b%10)*(a%10-b%10);
}

char solve(const char *s)
{
  int b[4], d[6];
  int i;

  sscanf(s, "%d,%d,%d,%d", b, b+1, b+2, b+3);
  d[0] = dist(b[0], b[1]);
  d[1] = dist(b[0], b[2]);
  d[2] = dist(b[0], b[3]);
  d[3] = dist(b[1], b[2]);
  d[4] = dist(b[1], b[3]);
  d[5] = dist(b[2], b[3]);
  qsort(d, 6, sizeof(int), cmp_int);
  for (i = 0; i < lut_size; i++) {
    if (memcmp(d, lut[i].d, sizeof(int)*6) == 0)
      return lut[i].c;
  }
  return '-';
}