求逆序数 + 离散化

POJ 2299 Ultra-QuickSort

裸逆序数对~

离散化后，用树状数组统计 “（在a[i]之前加入的） 比a[i]大的数 ” 的个数

贴个模板：

#include<map>
#include<set>
#include<list>
#include<cmath>
#include<ctime>
#include<queue>
#include<stack>
#include<cctype>
#include<cstdio>
#include<string>
#include<vector>
#include<cstdlib>
#include<cstring>
#include<complex>
#include<utility>    //pair
#include<iostream>
#include<algorithm>
#define MAXN 500005
#define INF 0x3f3f3f3f
#define LL long long
#define DBL double
#define eps 1e-6
#define pi acos(-1.0)
#define Test() cout<<"Test"<<endl;
#define Debug(a) cout<<#a<<" = "<<a<<endl;
#define Debug2(a,b) cout<<#a<<" = "<<a<<" , "<<#b<<" = "<<b<<endl;
using namespace std;

int n;
int a[MAXN], b[MAXN], c[MAXN];

int f(int x){
    int l=1, r=n, m;
    while(l<=r){
        m = (l+r)/2;
        if(x==b[m])    return m;
        if(x<b[m])    r=m-1;
        else    l=m+1;
    }
    return 0;
}
int lowbit(int x){
    return x&(-x);
}
void add(int p, int x){
    for(int i=p; i<=n; i+=lowbit(i))
        c[i]+=x;
}
int sum(int p){
    int s=0;
    for(int i=p; i>0; i-=lowbit(i))
        s+=c[i];
    return s;
}

int main()
{    
    while(cin >> n, n){
        for(int i=1; i<=n; i++){
            scanf("%d", &a[i]);
            b[i] = a[i];
        }
        sort(b+1, b+1+n);
        memset(c, 0, sizeof(c));
        LL ans=0;
        for(int i=1; i<=n; i++){
            int rank = f(a[i]);
            add(rank, 1);
            ans += (LL)(sum(n)-sum(rank));
        }
        cout << ans << endl;
    }
    
    return 0;
}