POJ2299 Ultra-QuickSort

Posted 2020-08-09

题解:

求逆序数裸题，

求逆序数，也就是把值放入树状数组中,而不是位置

值可能比较大，需要hash

代码:

#include<iostream>
#include<cstring>
#include<cstdio>
#include<algorithm>
#include<map>
using namespace std;
#define pb push_back
#define mp make_pair
#define se second
#define fs first
#define LL long long
#define CLR(x) memset(x,0,sizeof x)
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1 
typedef pair<int,int> P;
const double eps=1e-9;
const int maxn=5010;
const int mod=1e9+7;
const int INF=1e9;

int a[501000],b[501000],c[501000];
map<int,int> m;
int n;

int lowbit(int x){return x&-x;}

void Add(int x,int v){
    while(x<=n){
        c[x]+=v;
        x+=lowbit(x);
    }
}

int Sum(int x){
    int sum=0;
    while(x){
        sum+=c[x];
        x-=lowbit(x);
    }
    return sum;
}

int main(){
    while(~scanf("%d",&n)&&n){
        m.clear();
        for(int i=1;i<=n;i++){
            scanf("%d",&a[i]);
            m[a[i]]=i;
        }
        sort(a+1,a+n+1);
        for(int i=1;i<=n;i++) b[m[a[i]]]=i;
        CLR(c);
        LL sum=0;
        for(int i=1;i<=n;i++){
            sum+=Sum(n)-Sum(b[i]);
            Add(b[i],1);
        }
        printf("%lld\n",sum);
    }
    return 0;
}