hdu 1394 Minimum Inversion Number 树状数组+递推

来源：互联网发布：windows怎样编辑照片编辑：程序博客网时间：2024/06/05 01:14

水一道~

nlogn算出逆序数，再递推得到后面的答案，公式很容易推导。

#include<cstdio>#include<cstring>#include<algorithm>using namespace std;int c[5005];int a[5005];int ans,n;int lowbit(int x){     return x&(-x);}int Sum(int end){    int sum=0;    while(end>0)    {         sum+=c[end];         end=end-lowbit(end);    }    return sum;}void change(int i,int x){     while(i<=n)     {          c[i]=c[i]+x;          i=i+lowbit(i);     }}int main(){    while(scanf("%d",&n)!=EOF)    {        int tmp=0;        int s;        for(int i=1;i<=n;i++)        {            c[i]=0;            scanf("%d",&s);            a[i]=s+1;        }        for(int i=1;i<=n;i++)        {            change(a[i],1);            tmp+=(i-Sum(a[i]));        }        ans=tmp;        int now;        for(int i=1;i<n;i++)        {            now=tmp+(n+1-2*a[i]);            tmp=now;            ans=min(ans,now);        }        printf("%d\n",ans);    }    return 0;}