hdu 1394 Minimum Inversion Number 线段树

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

#include <cstdio>
#include <algorithm>
using namespace std;
#define lson l , m , rt << 1
#define rson m + 1 , r , rt << 1 | 1
const int maxn = 5555;
int sum[maxn<<2];
void PushUP(int rt) {
    sum[rt] = sum[rt<<1] + sum[rt<<1|1];
}
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 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);
    PushUP(rt);
}
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 x[maxn];
int main() {
    int n;
    while (~scanf("%d",&n)) {
        build(0 , n - 1 , 1);
        int sum = 0;
        for (int i = 0 ; i < n ; i ++) {
            scanf("%d",&x[i]);
            sum += query(x[i] , n - 1 , 0 , n - 1 , 1);
            update(x[i] , 0 , n - 1 , 1);
        }
        int ret = sum;
        for (int i = 0 ; i < n ; i ++) {
            sum += n - x[i] - x[i] - 1;
            ret = min(ret , sum);
        }
        printf("%d\n",ret);
    }
    return 0;
}