Sicily 13981 Cow Baseball

二分查找即可，偷懶直接用了庫函數

#include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;

int num[1005];
int main()
{
    int n, sum = 0, i = 0;
    scanf("%d", &n); 
    while (i != n)
        cin >> num[i++]; 
    sort(num, num + n);
    for (int x = 0; x < n - 1; x++)
        for (int y = x + 1; y < n; y++)
        {
            sum += upper_bound(num + y, num + n,  (3 * num[y] - 2 * num[x])) - lower_bound(num + y, num + n,  (2 * num[y] - num[x]));
        }
    printf("%d\n", sum);
}