HDU 5792 World is Exploding

预处理出每个数字前面比它小的有几个，比他大的有几个，后面比他小的有几个，比他大的有几个。

先算出最多可能产生多少对，然后减去多加的就是答案。

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<vector>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<iostream>
using namespace std;
typedef long long LL;
const double pi=acos(-1.0),eps=1e-8;
void File()
{
    freopen("D:\\in.txt","r",stdin);
    freopen("D:\\out.txt","w",stdout);
}
inline int read()
{
    char c = getchar();  while(!isdigit(c)) c = getchar();
    int x = 0;
    while(isdigit(c)) { x = x * 10 + c - '0'; c = getchar(); }
    return x;
}

const int maxn=50000+10;
int n,a[maxn],b[maxn],c[maxn];
int Lmax[maxn],Lmin[maxn],Rmax[maxn],Rmin[maxn];

int lowbit(int x){return x&(-x);}
int sum(int p){ int res=0; while(p) res=res+c[p],p=p-lowbit(p); return res;}
void update(int p) { while(p<=n) c[p]=c[p]+1,p=p+lowbit(p); }

int main()
{
    while(~scanf("%d",&n))
    {
        for(int i=1;i<=n;i++) scanf("%d",&a[i]),b[i-1]=a[i];
        sort(b, b + n); int sz = unique(b, b + n) - b;
        for(int i=1;i<=n;i++) a[i]=lower_bound(b, b + sz, a[i])-b+1;
        memset(c,0,sizeof c); 
        for(int i=1;i<=n;i++) { Lmin[i]=sum(a[i]-1); Lmax[i]=sum(n)-sum(a[i]); update(a[i]); }
        LL S=0; for(int i=1;i<=n;i++) S=S+Lmax[i];
        LL ans=0; for(int i=1;i<=n;i++) ans=ans+(LL)(S-Lmax[i])*(LL)Lmin[i];
        memset(c,0,sizeof c); 
        for(int i=n;i>=1;i--){ Rmin[i]=sum(a[i]-1); Rmax[i]=sum(n)-sum(a[i]); update(a[i]); }
        for(int i=1;i<=n;i++) ans=ans-(LL)Rmax[i]*(LL)Rmin[i]-(LL)Rmin[i]*(LL)Lmin[i]-(LL)Rmax[i]*(LL)Lmax[i];
        printf("%lld\n",ans);
    }
    return 0;
}