feat : Solver.solve()

app/src/main/java/sudoku/Cell.java

@@ -72,6 +72,10 @@ public abstract class Cell {
 	}
 	*/
 
+	public boolean isEmpty() {
+		return this.symbolIndex == Cell.NOSYMBOL;
+	}
+
 	public boolean equalsValue(Cell otherCell) {
 		return otherCell.getSymbolIndex() == this.getSymbolIndex();
 	}