学步园

http://acm.hdu.edu.cn/showproblem.php?pid=1394 

线段树求逆序数，先求出初始数组的逆序数(n logn)，然后o(1)推出其他逆序数。因为输入为0--n-1，所以建树从0开始。

code:

#include <cstdlib>
#include <cctype>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <vector>
#include <string>
#include <iostream>
#include <sstream>
#include <set>
#include <queue>
#include <stack>
#include <fstream>
#include <iomanip>
#include <bitset>
#include <list>
#include <ctime>
using namespace std ;
#define SET(arr, what)  memset(arr, what, sizeof(arr))
#define FF(i, a)        for(i=0; i<a; i++)
#define SD(a)           scanf("%d", &a)
#define SSD(a, b)       scanf("%d%d", &a, &b)
#define SF(a)           scanf("%lf", &a)
#define SS(a)           scanf("%s", a)
#define SLD(a)          scanf("%lld", &a)
#define PF(a)           printf("%d\n", a)
#define PPF(a, b)       printf("%d %d\n", a, b)
#define SZ(arr)         (int)a.size()
#define SWAP(a,b)       a=a xor b;b= a xor b;a=a xor b;
#define read            freopen("in.txt", "r", stdin)
#define write            freopen("out.txt", "w", stdout)
#define MAX             1<<30
#define ESP             1e-5
#define lson            l, m, rt<<1|1
#define rson            m+1, r, (rt<<1)+2
template<class T> inline T sqr(T a){return a*a;}
template<class T> inline void AMin(T &a,T b){if(a==-1||a>b)a=b;}
template<class T> inline void AMax(T &a,T b){if(a<b)a=b;}
template<class T> inline T Min(T a,T b){return a>b?b:a;}
template<class T> inline T Max(T a,T b){return a>b?a:b;}
const int maxn = 5010 ;
int sum[maxn<<2] ;
int data[maxn] ;
void build(int l, int r, int rt){
    sum[rt] = 0 ;
    if(l==r)    return ;
    int m = (l + r) >> 1 ;
    build(lson) ;
    build(rson) ;
}
void update(int x, int l, int r, int rt){
    sum[rt] ++ ;
    if(l==r)    return ;
    int m = (l + r) >> 1 ;
    if(x<=m) update(x, lson) ;
    else  update(x, rson) ;
}
int query(int L, int R, int l, int r, int rt){
    if(L<=l&&r<=R)  return sum[rt] ;
    int m = (l + r) >> 1 ;
    int ret = 0 ;
    if(L<=m) ret += query(L, R, lson) ;
    if(R>m)  ret += query(L, R, rson) ;
    return ret ;
}
int main(){
    int n, i, j, ini, ans ;
    while(~SD(n)){
        build(0, n-1, 0) ;
        ini = 0 ;
        FF(i, n){
            SD(data[i]) ;
            ini += query(data[i], n, 0, n-1, 0) ;
            update(data[i], 0, n-1, 0) ;
        }
        ans = ini ;
        FF(i, n){
            ini += n - data[i] - data[i] - 1 ;
            ans = Min(ini, ans) ;
        }
        PF(ans) ;
    }
    return 0 ;
}