bzoj2058[Usaco2010 Nov]Cow Photographs 归并+逆序对

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具体不好形容。。
http://www.cnblogs.com/Sakits/p/5837039.html
顺便复习了一波归并排序。。

#include<cstdio>#include<algorithm>#include<cstring>#include<iostream>#include<vector>#define fo(i,a,b) for(int i=a;i<=b;i++)#define fd(i,a,b) for(int i=a;i>=b;i--)using namespace std;typedef long long ll;const int N=1e5+5;int n,m;int a[N],b[N],tmp[N];int pos[N];ll ans;inline void mergesort(int l,int r){    if (l==r)return;    int mid=(l+r)>>1;    int cnt=l,h1=l,h2=mid+1;    mergesort(l,mid);    mergesort(mid+1,r);    while (h1<=mid&&h2<=r)    {        while (b[h1]>b[h2])        {            tmp[cnt++]=b[h2];            h2++;            ans+=mid-h1+1;            if (h2>r)break;        }        tmp[cnt++]=b[h1];        h1++;    }    fo(i,h1,mid)tmp[cnt++]=b[i];    fo(i,h2,r)tmp[cnt++]=b[i];    fo(i,l,r)b[i]=tmp[i];}int main(){    scanf("%d",&n);    fo(i,1,n)    {        scanf("%d",&b[i]);        pos[b[i]]=i;    }    mergesort(1,n);    ll sum=ans;    fo(i,1,n)    {        sum=sum-(pos[i]-1)+(n-pos[i]);        ans=min(ans,sum);    }    printf("%lld\n",ans);}

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