public static int pairs(int k, List<Integer> a) {
    List<Integer> arr = new ArrayList<>(a);
    Collections.sort(arr);
    int leftptr = 0;
    int rightptr = 1;
    int pairs = 0;
    while(rightptr>leftptr && rightptr<arr.size()) {
        int diff = arr.get(rightptr)-arr.get(leftptr);
        if(diff == k) {
            pairs++;
            rightptr++;
        } else if(diff > k) {
            if(rightptr-leftptr > 1) leftptr++;
            else if(rightptr-leftptr == 1) {
                leftptr++;
                rightptr++;
            }
        } else {
            rightptr++;
        }
    }
    return pairs;
}