refactor isValid

app/src/main/java/sudoku/structure/MultiDoku.java

@@ -112,11 +112,11 @@ public class MultiDoku {
 	 * @return boolean, true s'il est valide et false sinon.
 	 */
 	public boolean isValid() {
-		boolean result = true;
 		for (Sudoku sudoku : this.subGrids) {
-			result = sudoku.isValid() && result;
+			if(!sudoku.isValid())
+				return false;
 		}
-		return result;
+		return true;
 	}
 
 	@Override