Support expanding a uncovered tile surrounded by unflagged tiles.

src/Board.java

@@ -115,6 +115,21 @@ public class Board extends JPanel {
     setVisible(true);
   }
 
+  public int getTileFlaggedNum(int row, int col) {
+    int counter = 0;
+    for (int i = row - 1; i <= row + 1; i += 1) {
+      for (int j = col - 1; j <= col + 1; j += 1) {
+        if (!(i < 0 || j < 0 || i >= this.height || j >= this.width) && !(i == row && j == col)) {
+          Tile tile = this.tiles[i][j];
+          if (tile.isFlagged()) {
+            counter += 1;
+          }
+        }
+      }
+    }
+    return counter;
+  }
+
   public void showMines() {
     setVisible(false);
     for (int i = 0; i < this.height; i += 1) {

src/Tile.java

@@ -169,6 +169,9 @@ public class Tile extends JLabel implements MouseListener {
     }
     if (SwingUtilities.isLeftMouseButton(e) && !this.uncovered && !this.flagged) {
       this.setTileIcon("0");
+    } else if (this.isUncovered() && e.getClickCount() == 2 && SwingUtilities.isLeftMouseButton(e) &&
+               this.getValue() == this.board.getTileFlaggedNum(row, col)) {
+      this.board.expand(row, col);
     } else if (SwingUtilities.isMiddleMouseButton(e)) {
       if (!this.flagged) {
         this.setTileIcon("0");