1009. Triple Inversions (35)（树状数组求逆序数）

1009. Triple Inversions (35)

和1010一样，考察树状数组求逆序数，注意int可能溢出，用long long (2,3测试点)

#include <bits/stdc++.h>
using namespace std;
const int sz=100000+1024;
int c[sz],a[sz],n;
int ltb[sz],rts[sz];
void add(int i){
	while(i<=n){
		c[i]++;
		i+=-i&i;
	}
}
int get(int i){
	int sum=0;
	while(i>=1){
		sum+=c[i];
		i-=-i&i;
	}
	return sum;
}
int main()
{
	scanf("%d",&n);
	for(int i=1;i<=n;++i){
		scanf("%d",a+i);
	}
	for(int i=1;i<=n;++i){
		ltb[i]=i-get(a[i])-1;
		add(a[i]);
	}
	fill(c+1,c+n+1,0);
	for(int i=n;i>=1;--i){
		rts[i]=get(a[i]);
		add(a[i]);
	}
	long long res=0;
	for(int i=1;i<=n;++i){
		res+=(long long)ltb[i]*rts[i];
	}
	printf("%lld\n",res);
	return 0;
}