PAT T1009 Triple Inversions

树状数组判断三元逆序对~

#include<bits/stdc++.h>
using namespace std;
const int maxn=1e6+14;
int a[maxn];
int c[maxn*8];
long long l[maxn],r[maxn];
int lowbit (int n) {
    return n&-n;
}
int main () {
    int N;
    scanf ("%d",&N);
    for (int i=0;i<N;i++) {
        scanf ("%d",&a[i]);
        a[i]+=10010;
    }
    for (int i=0;i<N;i++) {
        for (int j=a[i]+1;j<maxn;j+=lowbit(j)) 
        l[i]+=c[j];
        for (int j=a[i];j>0;j-=lowbit(j)) 
        c[j]++;
    }
    memset (c,0,sizeof(c));
    for (int i=N-1;i>0;i--) {
        for (int j=a[i]-1;j>0;j-=lowbit(j)) 
        r[i]+=c[j];
        for (int j=a[i];j<maxn;j+=lowbit(j))
        c[j]++;
    }
    long long ans=0;
    for (int i=0;i<N;i++) 
    ans+=(long long)l[i]*r[i];
    printf ("%lld",ans);
    return 0;
}