public static int sockMerchant(int n, List<Integer> ar) {
        Collections.sort(ar);
        int counter = 0;
        Stack<Integer> stack = new Stack<>();
        
        for (Integer value : ar) {
            if(stack.empty()){
                stack.add(value);
            } else {
                if(stack.peek() == value){
                    stack.pop();
                    counter++;
                } else {    
                    stack.add(value);
                }
            }
        }
        return counter;
    }