BZOJ 3176 Sort

先一遍reverse+逆序对个数。

要开long long啊。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxn 100500
using namespace std;
int n,a[maxn],pos[maxn],l,bit[maxn];
long long ans=0;
int lowbit(int x) {return (x&(-x));}
void rev(int l,int r)
{
    for (int i=l;i<=r;i++)
    {
        if ((i<<1)>=l+r) break;
        swap(a[i],a[l+r-i]);
    }
}
int ask(int now)
{
    int ret=0;
    for (int i=now;i>=1;i-=lowbit(i))
        ret+=bit[i];
    return ret;
}
void insert(int now,int val)  
{
    for (int i=now;i<=n;i+=lowbit(i))
        bit[i]+=val;
}
int main()
{
    scanf("%d",&n);
    for (int i=1;i<=n;i++) scanf("%d",&a[i]);
    l=1;
    for (int i=2;i<=n;i++)
    {
        if (a[i]>a[i-1]) 
        {
            rev(l,i-1);
            l=i;ans++;    
        }
    } 
    rev(l,n);ans++;
    for (int i=1;i<=n;i++) pos[a[i]]=i;
    for (int i=n;i>=1;i--)  
    {
        ans+=(long long)ask(pos[i]-1);
        insert(pos[i],1);
    }
    printf("%lld\n",ans);
    return 0;
}
//Flaze naive!