HDU 1394 Minimum Inversion Number

求逆序数，然后o（1）推导其他的

/* ***********************************************
Author        :Zhou Zhentao
Email         :774388357@qq.com
Created Time  :2015/11/20 17:21:35
File Name     :acm.cpp
************************************************ */

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <string>
#include <math.h>
#include <stdlib.h>
#include <time.h>
using namespace std;

const int INF=0x7FFFFFFF;
#define lson l , m , rt << 1    
#define rson m + 1 , r , rt << 1 | 1   
#define root 1 , N , 1   
#define LL long long    
const int maxn = 50000+10;    
int a[maxn];
LL add[maxn<<2];    
LL sum[maxn<<2];    
void PushUp(int rt) {    
    sum[rt] = sum[rt<<1] + sum[rt<<1|1];    
}    
void PushDown(int rt,int m) {    
    if (add[rt]) {    
        add[rt<<1] += add[rt];    
        add[rt<<1|1] += add[rt];    
        sum[rt<<1] += add[rt] * (m - (m >> 1));    
        sum[rt<<1|1] += add[rt] * (m >> 1);    
        add[rt] = 0;    
    }    
}    
void build(int l,int r,int rt) {    
    add[rt] = 0;    
    if (l == r) {    
        scanf("%lld",&sum[rt]);    
        return ;    
    }    
    int m = (l + r) >> 1;    
    build(lson);    
    build(rson);    
    PushUp(rt);    
}    
void update(int L,int R,int c,int l,int r,int rt) {    
    if (L <= l && r <= R) {    
        add[rt] += c;    
        sum[rt] += (LL)c * (r - l + 1);    
        return ;    
    }    
    PushDown(rt , r - l + 1);    
    int m = (l + r) >> 1;    
    if (L <= m) update(L , R , c , lson);    
    if (m < R) update(L , R , c , rson);    
    PushUp(rt);    
}    
LL query(int L,int R,int l,int r,int rt) {    
    if (L <= l && r <= R) {    
        return sum[rt];    
    }    
    PushDown(rt , r - l + 1);    
    int m = (l + r) >> 1;    
    LL ret = 0;    
    if (L <= m) ret += query(L , R , lson);    
    if (m < R) ret += query(L , R , rson);    
    return ret;    
}    
int main()
{    
    int N;
    while(~scanf("%d",&N))
    {
        memset(add,0,sizeof add);
        memset(sum,0,sizeof sum);
        int k=0,ans=INF;
        for(int i=1;i<=N;i++)
        {
            scanf("%d",&a[i]);a[i]++;
            update(a[i] , a[i] , 1 , root); 
            if(a[i]==1) continue;
            k=k+(a[i]-1-query(1 , a[i]-1 ,root));
        }
        if(k<ans) ans=k;
        for(int i=1;i<=N-1;i++)
        {
            k=k-(a[i]-1)+(N-a[i]);
            if(k<ans) ans=k;
        }
        printf("%d\n",ans);
    }
    return 0;    
}