feat: add solutions to lc no.0840

Author: AE-Hertz

solution/0800-0899/0840.Magic Squares In Grid/Solution.js

@@ -0,0 +1,40 @@
+/**
+ * @param {number[][]} grid
+ * @return {number}
+ */
+var numMagicSquaresInside = function(grid) {
+    const isMagic = (i, j) => {
+        const s = new Set();
+        for (let x = 0; x < 3; x++) {
+            for (let y = 0; y < 3; y++) {
+                const num = grid[i + x][j + y];
+                if (num < 1 || num > 9 || s.has(num)) return false;
+                s.add(num);
+            }
+        }
+
+        const rowSum = grid[i][j] + grid[i][j+1] + grid[i][j+2];
+        if (rowSum !== grid[i+1][j] + grid[i+1][j+1] + grid[i+1][j+2]) return false;
+        if (rowSum !== grid[i+2][j] + grid[i+2][j+1] + grid[i+2][j+2]) return false;
+
+        const colSum1 = grid[i][j] + grid[i+1][j] + grid[i+2][j];
+        const colSum2 = grid[i][j+1] + grid[i+1][j+1] + grid[i+2][j+1];
+        const colSum3 = grid[i][j+2] + grid[i+1][j+2] + grid[i+2][j+2];
+        if (rowSum !== colSum1 || rowSum !== colSum2 || rowSum !== colSum3) return false;
+
+        if (rowSum !== grid[i][j] + grid[i+1][j+1] + grid[i+2][j+2]) return false;
+        if (rowSum !== grid[i][j+2] + grid[i+1][j+1] + grid[i+2][j]) return false;
+
+        return true;
+    };
+
+    let count = 0;
+
+    for (let i = 0; i < grid.length - 2; i++) {
+        for (let j = 0; j < grid[0].length - 2; j++) {
+            if (isMagic(i, j)) count++;
+        }
+    }
+
+    return count;
+};