poj2299

归并排序

#include<cstdio>
#include<cmath>
#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdlib>
#define maxn 510000
using namespace std;
long long n,a[maxn],ans,b[maxn];
long long read(){
    long long x=0,f=1;char ch=getchar();
    while (ch>'9' || ch<'0'){if (ch=='-') f=-1;ch=getchar();}
    while (ch>='0' && ch<='9'){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}
void merge(int l,int m,int r){
    int i=l,j=m+1,k=i;
    while (i<=m && j<=r){
        if (a[i]>a[j]){
            ans+=m-i+1;
            b[k++]=a[j++];
        }
        else b[k++]=a[i++];
    }
    while (i<=m) b[k++]=a[i++];
    while (j<=r) b[k++]=a[j++];
    for (int w=l;w<=r;w++) a[w]=b[w];
}
void merge_sort(int l,int r){
    if (l<r){
        int m=(l+r)>>1;
        merge_sort(l,m);
        merge_sort(m+1,r);
        merge(l,m,r);
    }
}
int main(){
    while (1){
        n=read();
        if (n==0) return 0;
        ans=0;
        memset(a,0,sizeof(a));
        memset(b,0,sizeof(b));
        for (int i=1;i<=n;i++) a[i]=read();
        merge_sort(1,n);
        printf("%lld\n",ans);
    }
}

树状数组

#include<cstdio>
#include<cmath>
#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdlib>
#define maxn 510000
#define ll long long
using namespace std;
struct node{
    ll data,rank;
}a[maxn];
ll n,k,b[maxn],cnt,sum[maxn];
bool cmp(node a,node b){
    return a.data<=b.data;
}
ll lowbit(ll x){return x&(-x);}
void update(ll x,ll d){
    for (int i=x;i<=k;i+=lowbit(i))
        sum[i]+=d;
}
ll getsum(ll x){
    ll tmp=0;
    for (int i=x;i;i-=lowbit(i))
        tmp+=sum[i];
    return tmp;
}
int main(){
    while (1){
        scanf("%lld",&k);
        if (k==0) return 0;
        memset(sum,0,sizeof(sum));
        ll ans=0,cnt=0;a[0].data=2100000000;
        for (int i=1;i<=k;i++){
            scanf("%lld",&a[i].data);
            a[i].rank=i;
        }
        sort(a+1,a+k+1,cmp);//离散化
        for (int i=1;i<=k;i++)
            b[a[i].rank]=a[i].data==a[i-1].data?cnt:++cnt;
        for (int i=1;i<=k;i++){
            update(b[i],1);
            ans+=i-getsum(b[i]);
        }
        printf("%lld\n",ans);
    }
}