L-Trominos

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define IMP {cout << "IMPOSSIBLE\n"; exit(0);}
char res[100][100];
char gvc(int x, int y) {
	x %= 5; y %= 5;
	return 'A'+(x*5+y);
}
void mk2x3(int x, int y) {
	char A = gvc(x, y), B = gvc(x+1, y+2);
	res[x][y] = A;
	res[x+1][y] = A;
	res[x][y+1] = A;
	res[x][y+2] = B;
	res[x+1][y+1] = B;
	res[x+1][y+2] = B;
}
void mk3x2(int x, int y) {
	char A = gvc(x, y), B = gvc(x+2, y+1);
	res[x][y] = A;
	res[x][y+1] = A;
	res[x+1][y] = A;
	res[x+2][y] = B;
	res[x+1][y+1] = B;
	res[x+2][y+1] = B;
}
void mk5x9(int x, int y) {
	mk3x2(x, y);
	mk2x3(x+3, y);
	mk2x3(x, y+6);
	mk3x2(x+2, y+5);
	mk3x2(x+2, y+7);
	char X = gvc(x, y+2);
	res[x][y+2] = X;
	res[x+1][y+2] = X;
	res[x][y+3] = X;
	X = gvc(x, y+4);
	res[x][y+4] = X;
	res[x][y+5] = X;
	res[x+1][y+5] = X;
	X = gvc(x+1, y+3);
	res[x+1][y+3] = X;
	res[x+1][y+4] = X;
	res[x+2][y+4] = X;
	X = gvc(x+2, y+2);
	res[x+2][y+2] = X;
	res[x+2][y+3] = X;
	res[x+3][y+3] = X;
	X = gvc(x+3, y+4);
	res[x+3][y+4] = X;
	res[x+4][y+4] = X;
	res[x+4][y+3] = X;
}
void mknx9(int n) {
	if (n == 3) IMP;
	mk5x9(0, 0);
	for (int i = 5; i < n; i += 2) {
		mk2x3(i, 0);
		mk2x3(i, 3);
		mk2x3(i, 6);
	}
}
int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int n, m;
	cin >> n >> m;
	bool trp = 0;
	if (m % 3 != 0) trp = 1, swap(n, m);
	if (m % 3 != 0) IMP
	if (n == 1) IMP
	if (n % 2 == 0) {
		for (int i = 0; i < n; i += 2) {
			for (int j = 0; j < m; j += 3) {
				mk2x3(i, j);
			}
		}
	} else if (m % 2 == 0) {
		for (int i = 0; i < m; i += 2) {
			mk3x2(0, i);
		}
		for (int i = 3; i < n; i += 2) {
			for (int j = 0; j < m; j += 3) {
				mk2x3(i, j);
			}
		}
	} else if (m == 3) IMP
	else {
		// m = 9+6k
		mknx9(n);
		for (int i = 9; i < m; i += 2) {
			mk3x2(0, i);
		}
		for (int i = 3; i < n; i += 2) {
			for (int j = 9; j < m; j += 3) {
				mk2x3(i, j);
			}
		}
	}
	if (!trp)
	for (int i = 0; i < n; ++i) {
		for (int j = 0; j < m; ++j) {
			cout << res[i][j];
		}
		cout << "\n";
	}
	else
	for (int j = 0; j < m; ++j) {
		for (int i = 0; i < n; ++i) {
			cout << res[i][j];
		}
		cout << "\n";
	}
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED

Test 24

Verdict: ACCEPTED

Test 25

Verdict: ACCEPTED

Test 26

Verdict: ACCEPTED

Test 27

Verdict: ACCEPTED

Test 28

Verdict: ACCEPTED

Test 29

Verdict: ACCEPTED

Test 30

Verdict: ACCEPTED

Test 31

Verdict: ACCEPTED

Test 32

Verdict: ACCEPTED

Test 33

Verdict: ACCEPTED

Test 34

Verdict: ACCEPTED

Test 35

Verdict: ACCEPTED

Test 36

Verdict: ACCEPTED

Test 37

Verdict: ACCEPTED

Test 38

Verdict: ACCEPTED

Test 39

Verdict: ACCEPTED

Test 40

Verdict: ACCEPTED

Test 41

Verdict: ACCEPTED

Test 42

Verdict: ACCEPTED

Test 43

Verdict: ACCEPTED

Test 44

Verdict: ACCEPTED

Test 45

Verdict: ACCEPTED

Test 46

Verdict: ACCEPTED

Test 47

Verdict: ACCEPTED

Test 48

Verdict: ACCEPTED

Test 49

Verdict: ACCEPTED

Test 50

Verdict: ACCEPTED

Test 51

Verdict: ACCEPTED

Test 52

Verdict: ACCEPTED

Test 53

Verdict: ACCEPTED

Test 54

Verdict: ACCEPTED

Test 55

Verdict: ACCEPTED

Test 56

Verdict: ACCEPTED

Test 57

Verdict: ACCEPTED

Test 58

Verdict: ACCEPTED

Test 59

Verdict: ACCEPTED

Test 60

Verdict: ACCEPTED

Test 61

Verdict: ACCEPTED

Test 62

Verdict: ACCEPTED

Test 63

Verdict: ACCEPTED

Test 64

Verdict: ACCEPTED

Test 65

Verdict: ACCEPTED

Test 66

Verdict: ACCEPTED

Test 67

Verdict: ACCEPTED

Test 68

Verdict: ACCEPTED

Test 69

Verdict: ACCEPTED

Test 70

Verdict: ACCEPTED

Test 71

Verdict: ACCEPTED

Test 72

Verdict: ACCEPTED

Test 73

Verdict: ACCEPTED

Test 74

Verdict: ACCEPTED

Test 75

Verdict: ACCEPTED

Test 76

Verdict: ACCEPTED

Test 77

Verdict: ACCEPTED

Test 78

Verdict: ACCEPTED

Test 79

Verdict: ACCEPTED

Test 80

Verdict: ACCEPTED

Test 81

Verdict: ACCEPTED

Test 82

Verdict: ACCEPTED

Test 83

Verdict: ACCEPTED

Test 84

Verdict: ACCEPTED

Test 85

Verdict: ACCEPTED

Test 86

Verdict: ACCEPTED

Test 87

Verdict: ACCEPTED

Test 88

Verdict: ACCEPTED

Test 89

Verdict: ACCEPTED

Test 90

Verdict: ACCEPTED

Test 91

Verdict: ACCEPTED