树状数组求逆序对

P1908 逆序对

 

 离散化+树状数组：AC_Code

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn=500010;

int a[maxn],tree[maxn<<2],b[maxn];
int cnt;

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

void updata(int i,int k){
    while( i<=cnt ){
        tree[i]+=k;
        i+=lowbit(i);
    }
}

ll getsum(int i){
    ll res=0;
    while( i>0 ){
        res+=tree[i];
        i-=lowbit(i);
    }
    return res;
}

int main()
{
    int n;
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        scanf("%d",&a[i]);
        b[i-1]=a[i];
    }
    sort(b,b+n);
    int reu=unique(b,b+n)-b;
    cnt=reu;
    memset(tree,0,sizeof(tree));
    ll ans=0;
    for(int i=1;i<=n;i++){
        int f=lower_bound(b,b+reu,a[i])-b+1;
        ll t=getsum(cnt)-getsum(f);        //t=getsum(cnt)-getsum(a[i]);
        updata(f,1);
        ans += t;
    }
    cout<<ans<<endl;
    return 0;
}