拼多多 第一题，第三题，求帮忙看看哪错啦？

public static void main(String[] args) {
    Scanner sc=new Scanner(System.in);

    String nn=sc.nextLine(); if(nn==null||nn.length()==0){
        System.out.println(0);
    } int n=new Integer(nn); if(n==0||n<4||n%2!=0)
        System.out.println(0);

    String str=sc.nextLine();
    String[] strings=str.split(" "); int[] arr=new int[strings.length]; for(int i=0;i<strings.length;i++){
        arr[i]=new Integer(strings[i]);
    }
    Arrays.sort(arr); int max=arr[0]+arr[n-1]; int min=arr[n/2-1]+arr[n/2];
    System.out.println(max-min);
}

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in); //String str = "{31,18,19,1,25}";  int d = 10; double res = 0;

    String str = sc.next(); if(str!=null&&str.length()>2){
        str=str.substring(1,str.length()-1);
        String[] strings=str.split(","); int[] arr=new int[strings.length]; for(int i=0;i<strings.length;i++){
            arr[i]=Integer.valueOf(strings[i]);
        }
        d = sc.nextInt();

        PDD3 obj = new PDD3();
        obj.combinationSum(arr, 2, d);
        res = (double) count / total;
        System.out.println(String.format("%.6f", res));
    }else{
        System.out.println(String.format("%.6f", res));
    }




} static int count = 0; static int total = 0; public List<List<Integer>> combinationSum(int[] candidates, int target, int d) {
    List<List<Integer>> result = new ArrayList<>();
    Arrays.sort(candidates);
    backTrack(candidates, result, new ArrayList<Integer>(), target, 0, d); return result;
} private void backTrack(int[] candidates, List<List<Integer>> result, ArrayList<Integer> list, int remain, int start, int d) { if (remain == 0) { if (Math.abs(list.get(0) - list.get(1)) <= d) { count++;
        } total++;
        result.add(new ArrayList<>(list));
    } else { for (int i = start; i < candidates.length; i++) {
            list.add(candidates[i]);
            backTrack(candidates, result, list, remain - 1, i + 1, d);
            list.remove(list.size() - 1);
        }
    }
}