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

一开始以为死于精度……调了半天发现死于long long……

一、二分法：

#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;

}

二、尺取法：

#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;

}