【归并排序求逆序对个数】【树状数组求逆序对个数】

【AC】

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;

const int maxn=1e5+2;
int a[maxn];
int tmp[maxn];
int n;
ll ans;
void Merge(int l,int m,int r)
{
    int i=l,j=m+1;
    int k=l;
    while(i<=m&&j<=r)
    {
        if(a[i]<=a[j])
        {
            tmp[k++]=a[i++];
        }
        else 
        {
            tmp[k++]=a[j++];
            ans+=(ll)(m-i+1);
        }
    }
    while(i<=m) tmp[k++]=a[i++];
    while(j<=r) tmp[k++]=a[j++];
    for(int i=l;i<=r;i++)
    {
        a[i]=tmp[i];
    } 
}

void Merge_Sort(int l,int r)
{
    if(l<r)
    {
        int mid=(l+r)>>1;
        Merge_Sort(l,mid);
        Merge_Sort(mid+1,r);
        Merge(l,mid,r);    
    }    
}
int main()
{
    while(~scanf("%d",&n))
    {
        for(int i=0;i<n;i++)
        {
            scanf("%d",&a[i]);
        }
        ans=0;
        Merge_Sort(0,n-1);
        printf("%lld
",ans);
    }
    return 0;
}

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
#include<algorithm>
using namespace std;
typedef long long ll;
const int maxn=5e5+2;

int n;
ll ans;
struct node
{
    int x;
    int pos;    
}a[maxn];
int tree[maxn];
int r[maxn];
bool cmp(node nd1,node nd2)
{
    return nd1.x<nd2.x;
}
int lowbit(int x)
{
    return x&-x;
}
void add(int k,int x)
{
    while(k<=n)
    {
        tree[k]+=x;
        k+=lowbit(k);
    }
}
ll query(int k)
{
    ll ans=0;
    while(k)
    {
        ans+=tree[k];
        k-=lowbit(k);
    }
    return ans;
}
int main()
{
    while(scanf("%d",&n)&&n)
    {
        for(int i=1;i<=n;i++)
        {
            scanf("%d",&a[i].x);
            a[i].pos=i;
        }
        sort(a+1,a+n+1,cmp); 
        for(int i=1;i<=n;i++)
        {
            r[a[i].pos]=i;
        }
        memset(tree,0,sizeof(tree));
        ans=0;
        for(int i=1;i<=n;i++)
        {
            add(r[i],1);
            ans+=i-query(r[i]);
        }
        cout<<ans<<endl;
    }
    return 0;
}