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+;
int a[maxn];
int c[maxn*];
long long l[maxn],r[maxn];
int lowbit (int n) {
    return n&-n;
}
int main () {
    int N;
    scanf ("%d",&N);
    for (int i=;i<N;i++) {
        scanf ("%d",&a[i]);
        a[i]+=;
    }
    for (int i=;i<N;i++) {
        for (int j=a[i]+;j<maxn;j+=lowbit(j))
        l[i]+=c[j];
        for (int j=a[i];j>;j-=lowbit(j))
        c[j]++;
    }
    memset (c,,sizeof(c));
    for (int i=N-;i>;i--) {
        for (int j=a[i]-;j>;j-=lowbit(j))
        r[i]+=c[j];
        for (int j=a[i];j<maxn;j+=lowbit(j))
        c[j]++;
    }
    long long ans=;
    for (int i=;i<N;i++)
    ans+=(long long)l[i]*r[i];
    printf ("%lld",ans);
    return ;
}