java: add «840. Magic Squares In Grid»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/magic-squares-in-grid.java

@@ -0,0 +1,68 @@
+class Solution {
+  private static final int SQUARE_SIZE = 3;
+
+  private int sum(int[][] grid, int y, int x, int dy, int dx) {
+    int total = 0;
+    for (int d = 0; d < SQUARE_SIZE; ++d) {
+      total += grid[y + d * dy][x + d * dx];
+    }
+    return total;
+  }
+
+  private static final int ALL_DIGITS = (~(((~0) >> 9) << 9));
+
+  private boolean isMagicSquare(int[][] grid, int y, int x) {
+    // check the digits
+    int seen = 0;
+    for (int d = 0; d < SQUARE_SIZE * SQUARE_SIZE; ++d) {
+      int dy = d / SQUARE_SIZE, dx = d % SQUARE_SIZE;
+
+      int digit = grid[y + dy][x + dx];
+      if (digit < 1 || digit > 9) {
+        // out of range
+        return false;
+      }
+
+      seen |= 1 << (digit - 1);
+    }
+
+    if (seen != ALL_DIGITS) {
+      // repeated digits
+      return false;
+    }
+
+    // check the sums
+    int expectedSum = sum(grid, y, x, 1, 1);
+    int diag2 = sum(grid, y, x + 2, 1, -1);
+    if (diag2 != expectedSum) {
+      // diagonals are broken
+      return false;
+    }
+
+    for (int d = 0; d < SQUARE_SIZE; ++d) {
+      if (sum(grid, y + d, x, 0, 1) != expectedSum) {
+        return false;
+      }
+
+      if (sum(grid, y, x + d, 1, 0) != expectedSum) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  public int numMagicSquaresInside(int[][] grid) {
+    int squares = 0;
+
+    for (int y = 0; y + 2 < grid.length; ++y) {
+      for (int x = 0; x + 2 < grid[y].length; ++x) {
+        if (isMagicSquare(grid, y, x)) {
+          ++squares;
+        }
+      }
+    }
+
+    return squares;
+  }
+}