public boolean canCross(int[] stones) {
        assert(stones != null);
        assert (stones.length > 0);

        // BFS
        //Queue<Integer> frontier = new LinkedBlockingQueue<>();

        // DFS
        Stack<Integer> frontier = new Stack<>();
        Set<Integer> visited = new HashSet<>();
        Set<Integer> stoneSet = Arrays.stream(stones).boxed().collect(Collectors.toSet());

        int targetStone = stones[stones.length - 1];

        // start our exploration from the very first stone
        frontier.add(stones[0]);

        while (!frontier.isEmpty()) {

            // take the next stone from the frontier
            int currentStone = frontier.pop();
            visited.add(currentStone);

            // end of the route
            if (currentStone == targetStone) {
                return true;
            }

            // expand the current state into frontier
            for (Integer step : STEPS) {
                int next = currentStone + step;
                if (stoneSet.contains(next)) {
                    if (!frontier.contains(next) &&
                            !visited.contains(next)) {
                        frontier.add(next);
                    }
                }
            }
        }

        return false;
    }