codeforence 251A：（二分）

选出三个数，让最大的数和最小的数之差不超过d~

那只需要将所有数排序后，找出<=（最小数+d）的数的位置，之间的数都能进行选择

#include"cstdio"
#include"cstring"
#include"algorithm"
#define MAXN 100005
using namespace std;
long long num[MAXN];
long long tail;
long long find(long long n)
{   long long low=0,high=tail-1,mid;
    while(high>=low)
    {   mid=low+(high-low)/2;
        if(num[mid]==n) return mid;
        else if(num[mid]>=n) high=mid-1;
        else low=mid+1;
    }
    return low-1;
}
int main()
{   long long d,n,ans;
    while(scanf("%lld %lld",&n,&d)!=EOF)
    {   tail=0;
        ans=0;
        for(long long i=0;i<n;i++)
            scanf("%lld",&num[i]);
        for(int i=0;i<n;)
        {   int k=i+1;
            while(num[i]==num[k]&&k<n) k++;
            num[tail++]=num[i];
            i=k;
        }
        sort(num,num+tail);
        for(long long i=0;i<tail;i++)
        {   long long p=find(d+num[i])-i-1;
            ans+=(1+p)*p/2;
        }
        printf("%lld\n",ans);
    }
    return 0;
}