Fix sorting comparator in CombinationSumTest

Author: Avaneeshakrishna

src/test/java/com/thealgorithms/backtracking/CombinationSumTest.java

@@ -16,7 +16,7 @@ private static List<List<Integer>> norm(List<List<Integer>> x) {
             q.sort(Integer::compare);
             y.add(q);
         }
-        y.sort(Comparator.<List<Integer>, Integer>comparing(List::size).thenComparing(a -> a.toString()));
+        y.sort(Comparator.comparingInt(List::size).thenComparing(Object::toString));
         return y;
     }