In question, it is asked to generate fibonacci series for n using memoization i.e.

public static List<Integer> fibonacci(int n);

whereas solution talks about generating n fibonacci using memoization i.e.

public static int fibonacci(int n);

The solution need to be updated as follows:

• Base case:
  • n = 0
    • return List.of(0);
  • n = 1
    • return List.of(0, 1);
  • n > 1
    • add 0 and 1 in result and recursive call to fibonacci generation.

public static List<Integer> fibonacci(int n) {
        if (n == 0) return List.of(0);
        if (n == 1) return List.of(0, 1);

        List<Integer> result = new ArrayList<>();
        result.add(0);
        result.add(1);
        Map<Integer, Integer> cache = new HashMap<>();
        fib(result, cache, n);
        return result;
    }

    public static int fib(List<Integer> result, Map<Integer, Integer> cache, int n) {
        if (cache.containsKey(n)) return cache.get(n);

        if (n <= 1) {
            cache.put(n, n);
            return n;
        }

        int f = fib(result, cache, n - 1) + fib(result, cache, n - 2);

        result.add(f);
        cache.put(n, f);

        return f;
    }