hdu_1394，线段树求逆序数

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http://www.notonlysuccess.com/index.php/segment-tree-complete/

#include<iostream>#include<cstdio>#include<cstring>#include<algorithm>#define lson l,m,rt<<1#define rson m+1,r,rt<<1|1using namespace std;const int maxn=5555;int sum[maxn<<2];int seq[maxn];void pushUp(int rt){    sum[rt]=sum[rt<<1]+sum[rt<<1|1];}void build(int l,int r,int rt){    if(r==l)    {        sum[rt]=0;        return;    }    int m=(r+l)>>1;    build(lson);    build(rson);}void update(int L,int d,int l,int r,int rt){    if(l==r)    {        sum[rt]=d;        return;    }    int m=(r+l)>>1;    if(L<=m) update(L,d,lson);    else update(L,d,rson);    pushUp(rt);}int query(int L,int R,int l,int r,int rt){    int ret=0;    if(L<=l&&r<=R){return sum[rt];}    int m=(l+r)>>1;    if(L<=m) ret+=query(L,R,lson);    if(R>m) ret+=query(L,R,rson);    return ret;}int main(){    int n;    while(scanf("%d",&n)!=EOF)    {        int ans=0;        memset(sum,0,sizeof(sum));        build(0,n-1,1);        memset(seq,0,sizeof(seq));        for(int i=0;i<n;i++)        {            scanf("%d",&seq[i]);            ans+=i-query(0,seq[i],0,n-1,1);            //cout<<ans<<" ";            //cout<<endl;            update(seq[i],1,0,n-1,1);        }        int cur=ans;        for(int i=0;i<n;i++)        {            cur+=n-seq[i]*2-1;            ans=min(ans,cur);        }        cout<<ans<<endl;    }    return 0;}

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