聪明的小羊肖恩（双指针）

题目

import java.util.Arrays;
import java.util.Scanner;

public class Main {
	public static long calc(long[] num,int max) {		
		int l = 0;
		int r = num.length-1;

		long res = 0;
		
	    while(l<r) {
			while(l<r && num[l]+num[r]>max) {
				r--;
			}
			res+=(r-l);
			l++;
		}
    
		return res;
	}
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int l = sc.nextInt();
		int r = sc.nextInt();
		sc.nextLine();
		long[] num = new long[n];
		for(int i=0;i<n;i++)
			num[i] = sc.nextLong();

		Arrays.sort(num);
	    //1 2 3

		long res1 = calc(num,r);
		long res2 = calc(num,l-1);
		//找l到r之间的就等价于找r减去（l-1）的

    System.out.println(res1-res2);
		sc.close();
	}
}