LeetCode-in-Java

840. Magic Squares In Grid

Medium

A 3 x 3 magic square is a 3 x 3 grid filled with distinct numbers from 1 to 9 such that each row, column, and both diagonals all have the same sum.

Given a row x col grid of integers, how many 3 x 3 “magic square” subgrids are there? (Each subgrid is contiguous).

Example 1:

Input: grid = [[4,3,8,4],[9,5,1,9],[2,7,6,2]]

Output: 1

Explanation: The following subgrid is a 3 x 3 magic square: while this one is not: In total, there is only one magic square inside the given grid.

Example 2:

Input: grid = [[8]]

Output: 0

Constraints:

• row == grid.length
• col == grid[i].length
• 1 <= row, col <= 10
• 0 <= grid[i][j] <= 15

Solution

import java.util.HashSet;
import java.util.Set;

public class Solution {
    public int numMagicSquaresInside(int[][] grid) {
        int m = grid.length;
        int n = grid[0].length;
        int count = 0;
        for (int i = 0; i < m - 2; i++) {
            for (int j = 0; j < n - 2; j++) {
                Set<Integer> set = new HashSet<>();
                int sum = grid[i][j] + grid[i][j + 1] + grid[i][j + 2];
                if (sum == grid[i + 1][j] + grid[i + 1][j + 1] + grid[i + 1][j + 2]
                        && sum == grid[i + 2][j] + grid[i + 2][j + 1] + grid[i + 2][j + 2]
                        && sum == grid[i][j] + grid[i + 1][j] + grid[i + 2][j]
                        && sum == grid[i][j + 1] + grid[i + 1][j + 1] + grid[i + 2][j + 1]
                        && sum == grid[i][j + 2] + grid[i + 1][j + 2] + grid[i + 2][j + 2]
                        && sum == grid[i][j] + grid[i + 1][j + 1] + grid[i + 2][j + 2]
                        && sum == grid[i][j + 2] + grid[i + 1][j + 1] + grid[i + 2][j]
                        && set.add(grid[i][j])
                        && isLegit(grid[i][j])
                        && set.add(grid[i][j + 1])
                        && isLegit(grid[i][j + 1])
                        && set.add(grid[i][j + 2])
                        && isLegit(grid[i][j + 2])
                        && set.add(grid[i + 1][j])
                        && isLegit(grid[i + 1][j])
                        && set.add(grid[i + 1][j + 1])
                        && isLegit(grid[i + 1][j + 1])
                        && set.add(grid[i + 1][j + 2])
                        && isLegit(grid[i + 1][j + 2])
                        && set.add(grid[i + 2][j])
                        && isLegit(grid[i + 2][j])
                        && set.add(grid[i + 2][j + 1])
                        && isLegit(grid[i + 2][j + 1])
                        && set.add(grid[i + 2][j + 2])
                        && isLegit(grid[i + 2][j + 2])) {
                    count++;
                }
            }
        }
        return count;
    }

    private boolean isLegit(int num) {
        return num <= 9 && num >= 1;
    }
}

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