HDU 1394 Minimum Inversion Number 线段树

#include <iostream>
#include <cstdio>
#include <string>
#include <vector>
#include <map>
#include <cstring>
#include <iterator>
#include <cmath>
#include <algorithm>
#include <stack>
#include <deque>
#include <map>

#define lson l, m, rt<<1
#define rson m+1, r, rt<<1|1

const int maxn = 5000+7;
using namespace std;
int sum[4*maxn];
int arr[maxn];

void Pushplus( int rt ) {
    sum[rt] = sum[rt<<1] + sum[rt<<1|1];
    return;
}

void build( int l, int r, int rt ) {
    sum[rt] = 0;
    if( l == r ) return;
    int m = (l + r) >> 1;
    build( lson );
    build( rson );
    return;
}

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;
}

void update(int p, int l, int r, int rt) {
    if( l == r ) {
        sum[rt]++;
        return;
    }
    int m = (l+r) >> 1;
    if( p <= m ) update(p, lson);
    else update(p, rson);
    Pushplus(rt);
}

int main() {
    int n;
    while( scanf( "%d", &n ) == 1 ) {
        build( 0, n-1, 1 );
        int sum = 0;
        for( int i = 0; i < n; i++ ) {
            scanf( "%d", arr + i );
            sum += Query( arr[i], n-1, 0, n-1, 1 );
            update(arr[i], 0, n-1, 1);
        }
        int ret = sum;
        for( int i = 0; i < n; i++ ) {
            sum += n - arr[i] - arr[i] - 1;
            ret = min( ret, sum );
        }
        printf( "%d\n", ret );
    }
}