static int countPairsWithDiffK(List<Integer> list,
                                   int k) {
        int count = 0;
        Collections.sort(list);
        int n = list.size();
        int l = 0;
        int r = 0;
        while (r < n) {
            if (list.get(r) - list.get(l) == k) {
                count++;
                l++;
                r++;
            } else if (list.get(r) - list.get(r) > k)
                l++;
            else
                r++;
        }
        return count;
    }