public boolean containsNearbyDuplicate(int[] nums, int k) {
        if (nums == null) return false;
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (int i = 0; i < nums.length; ++i) {
            if (map.containsKey(nums[i])) {
                if (i - map.get(nums[i]) <= k) return true;
            }
            map.put(nums[i], i);
        }
        return false;
    }