Add solution #840

Author: JoshCrozier

README.md

@@ -647,6 +647,7 @@
 837|[New 21 Game](./0837-new-21-game.js)|Medium|
 838|[Push Dominoes](./0838-push-dominoes.js)|Medium|
 839|[Similar String Groups](./0839-similar-string-groups.js)|Hard|
+840|[Magic Squares In Grid](./0840-magic-squares-in-grid.js)|Medium|
 841|[Keys and Rooms](./0841-keys-and-rooms.js)|Medium|
 844|[Backspace String Compare](./0844-backspace-string-compare.js)|Easy|
 846|[Hand of Straights](./0846-hand-of-straights.js)|Medium|

solutions/0840-magic-squares-in-grid.js

@@ -0,0 +1,56 @@
+/**
+ * 840. Magic Squares In Grid
+ * https://leetcode.com/problems/magic-squares-in-grid/
+ * Difficulty: 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?
+ *
+ * Note: while a magic square can only contain numbers from 1 to 9, grid may contain numbers up
+ * to 15.
+ */
+
+/**
+ * @param {number[][]} grid
+ * @return {number}
+ */
+var numMagicSquaresInside = function(grid) {
+  const rows = grid.length;
+  const cols = grid[0].length;
+
+  if (rows < 3 || cols < 3) return 0;
+
+  let result = 0;
+  for (let r = 0; r <= rows - 3; r++) {
+    for (let c = 0; c <= cols - 3; c++) {
+      if (isMagicSquare(grid, r, c)) result++;
+    }
+  }
+
+  return result;
+};
+
+function isMagicSquare(grid, r, c) {
+  if (grid[r + 1][c + 1] !== 5) return false;
+
+  const cells = [
+    grid[r][c], grid[r][c + 1], grid[r][c + 2],
+    grid[r + 1][c], grid[r + 1][c + 1], grid[r + 1][c + 2],
+    grid[r + 2][c], grid[r + 2][c + 1], grid[r + 2][c + 2]
+  ];
+
+  if (!cells.every(val => val >= 1 && val <= 9) || new Set(cells).size !== 9) return false;
+
+  return (
+    grid[r][c] + grid[r][c + 1] + grid[r][c + 2] === 15
+    && grid[r + 1][c] + grid[r + 1][c + 1] + grid[r + 1][c + 2] === 15
+    && grid[r + 2][c] + grid[r + 2][c + 1] + grid[r + 2][c + 2] === 15
+    && grid[r][c] + grid[r + 1][c] + grid[r + 2][c] === 15
+    && grid[r][c + 1] + grid[r + 1][c + 1] + grid[r + 2][c + 1] === 15
+    && grid[r][c + 2] + grid[r + 1][c + 2] + grid[r + 2][c + 2] === 15
+    && grid[r][c] + grid[r + 1][c + 1] + grid[r + 2][c + 2] === 15
+    && grid[r][c + 2] + grid[r + 1][c + 1] + grid[r + 2][c] === 15
+  );
+}