Niitty

#define print_board for (int i = 0; i < n; i++) { std::cout << board[i] << '\n'; } std::cout << std::endl
#define print_flowers(flowers) for (int i = 0; i < 26; i++) { std::cout << ((flowers & (1<<i)) >> i); } std::cout << '\n' << "abcdefghijklmnopqrstuvwxyz" << std::endl
#define print_rect(r) std::cout << r.left << ' ' << r.right << ' ' << r.top << ' ' << r.bottom << std::endl
#define to_bit(c) (1<<(c - 'a'))
#define MAX_RECT ((rect) { .left = 0, .right = (int16_t) (n-1), .top = 0, .bottom = (int16_t) (n-1) })
#define CENTER_RECT ((rect) { .left = (int16_t (n>>1)), .right = (int16_t (n>>1)), .top = (int16_t (n>>1)), .bottom = (int16_t (n>>1)) })
#define rect_size(r) ((1 + r.right - r.left) * (1 + r.bottom - r.top))
#define enclosed_by(a, b) (a.left >= b.left && a.right <= b.right && a.top >= b.top && a.bottom <= b.bottom)
#define bitmap uint32_t
 
#include <iostream>
#include <stdlib.h>
#include <stdint.h>
 
// note: int16_t only needed for last part!
// other parts can do with int8_t
typedef struct {
    int16_t left;
    int16_t right;
    int16_t top;
    int16_t bottom;
} rect;

inline bool valid_rect_nobiggest(char **board, uint16_t n, bitmap flowers, rect rectangle) {
    bitmap flowers_in_rect = 0;
    for (uint16_t x = rectangle.left; x <= rectangle.right; x++) {
        for (uint16_t y = rectangle.top; y <= rectangle.bottom; y++) {
            flowers_in_rect = flowers_in_rect | to_bit(board[x][y]);
            if (flowers == flowers_in_rect) { return true; }
        }
    }
    return false;
}

inline bool valid_rect(char **board, uint16_t n, bitmap flowers, rect biggest_invalid_rect, rect rectangle) {
    return (!enclosed_by(rectangle, biggest_invalid_rect)) && valid_rect_nobiggest(board, n, flowers, rectangle);
}
 
int main() {
    uint16_t n;
    bitmap found_flowers = 0;
    std::cin >> n;
 
    char **board = (char**) malloc((n) * sizeof(char*));
    
    for (unsigned int i = 0; i < n; i++) {
        char *row = (char*) malloc((n+1) * sizeof(char));
        std::cin >> row;
        for (unsigned int j = 0; j < n; j++) {
            found_flowers = found_flowers | to_bit(row[j]);
        }
        board[i] = row;
    }
 
    rect biggest_invalid_rect = CENTER_RECT;
    if (valid_rect_nobiggest(board, n, found_flowers, biggest_invalid_rect)) {
        biggest_invalid_rect.left = 501;
    }

    // naive solution for biggest invalid rect
    do { biggest_invalid_rect.left--; }
    while (biggest_invalid_rect.left >= 0 && !valid_rect_nobiggest(board, n, found_flowers, biggest_invalid_rect));
    biggest_invalid_rect.left++;

    do { biggest_invalid_rect.right++; }
    while (biggest_invalid_rect.right < n && !valid_rect_nobiggest(board, n, found_flowers, biggest_invalid_rect));
    biggest_invalid_rect.right--;
    
    do { biggest_invalid_rect.top--; }
    while (biggest_invalid_rect.top >= 0 && !valid_rect_nobiggest(board, n, found_flowers, biggest_invalid_rect));
    biggest_invalid_rect.top++;
    
    do { biggest_invalid_rect.bottom++; }
    while (biggest_invalid_rect.bottom < n && !valid_rect_nobiggest(board, n, found_flowers, biggest_invalid_rect));
    biggest_invalid_rect.bottom--;

    // brute force solution
    unsigned int solutions = 0;
    for (int16_t left = 0; left < n; left++) {
        for (int16_t right = left; right < n; right++) {
            for (int16_t top = 0; top < n; top++) {
                for (int16_t bottom = top; bottom < n; bottom++) {
                    if (valid_rect(board, n, found_flowers, biggest_invalid_rect, (rect) {.left=left, .right=right, .top=top, .bottom=bottom})) {
                        solutions++;
                    }
                }
            }
        }
    }
 
    std::cout << solutions << std::endl;
 
    /*
    for (unsigned int i = 0; i < n; i++) {
        free(board[i]);
    }
    free(board);
    */
 
    return 0;
}

Test 1

Group: 1, 2, 3, 4, 5, 6

Verdict: ACCEPTED

Test 2

Group: 1, 2, 3, 4, 5, 6

Verdict: ACCEPTED

Test 3

Group: 1, 2, 3, 4, 5, 6

Verdict: ACCEPTED

Test 4

Group: 1, 2, 3, 4, 5, 6

Verdict: ACCEPTED

Test 5

Group: 1, 2, 3, 4, 5, 6

Verdict: ACCEPTED

Test 6

Group: 2, 3, 4, 5, 6

Verdict: ACCEPTED

Test 7

Group: 2, 3, 4, 5, 6

Verdict: ACCEPTED

Test 8

Group: 2, 3, 4, 5, 6

Verdict: ACCEPTED

Test 9

Group: 2, 3, 4, 5, 6

Verdict: ACCEPTED

Test 10

Group: 3, 4, 5, 6

Verdict: ACCEPTED

Test 11

Group: 3, 4, 5, 6

Verdict: ACCEPTED

Test 12

Group: 3, 4, 5, 6

Verdict: ACCEPTED

Test 13

Group: 3, 4, 5, 6

Verdict: ACCEPTED

Test 14

Group: 4, 5, 6

Verdict: ACCEPTED

Test 15

Group: 4, 5, 6

Verdict: TIME LIMIT EXCEEDED

Test 16

Group: 4, 5, 6

Verdict: ACCEPTED

Test 17

Group: 4, 5, 6

Verdict: TIME LIMIT EXCEEDED

Test 18

Group: 5, 6

Verdict: TIME LIMIT EXCEEDED

Test 19

Group: 5, 6

Verdict: TIME LIMIT EXCEEDED

Test 20

Group: 5, 6

Verdict: TIME LIMIT EXCEEDED

Test 21

Group: 5, 6

Verdict: TIME LIMIT EXCEEDED

Test 22

Group: 6

Verdict: TIME LIMIT EXCEEDED

Test 23

Group: 6

Verdict: TIME LIMIT EXCEEDED

Test 24

Group: 6

Verdict: TIME LIMIT EXCEEDED

Test 25

Group: 6

Verdict: TIME LIMIT EXCEEDED