【二分法】【尺取法】bzoj2348 [Baltic 2011]Plagiarism

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
bool cmp(const int &a,const int &b){return a>b;}
int n,a[100001],b[100001];
long long ans;
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;++i) scanf("%d",&a[i]);
    sort(a+1,a+n+1,cmp);
    memcpy(b,a,(n+1)*sizeof(int));
    for(int i=1;i<=n;++i) b[i]*=10;
    for(int i=1;i<=n;++i) ans+=(long long)(upper_bound(b+i+1,b+n+1,a[i]*9,cmp)-b-i-1);
    printf("%lld\n",ans);
    return 0;
}

#include<cstdio>
#include<algorithm>
using namespace std;
bool cmp(const int &a,const int &b){return a>b;}
int n,a[100001];
long long ans;
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;++i) scanf("%d",&a[i]);
    sort(a+1,a+n+1,cmp);
    int head=1,tail=1;
    while(1)
      {
        while(a[tail]*10>=a[head]*9 && tail<=n) ++tail;
        if(head==n) break;
        ans+=(long long)(tail-head-1);
        ++head;
      }
    printf("%lld\n",ans);
    return 0;
}