cf 61 E. Enemy is weak　离散化＋树状数组

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <iostream>
#include <math.h>

#define LL long long

using namespace std;

const int maxn = 1000000 + 10;

struct Node
{
    int id,init,chg;
};
Node node[maxn];
int fir[maxn];
int sec[maxn];
int c[maxn];

bool cmp1(Node x,Node y)
{
    return x.init<y.init;
}

bool cmp2(Node x,Node y)
{
    return x.id<y.id;
}

inline int lb(int x)
{
    return x & (-x);
}

void update(int x,int add)
{
    while(x <= maxn){
        c[x] += add;
        x += lb(x);
    }
}

int query(int x)
{
    int ret = 0;
    while(x > 0){
        ret += c[x];
        x -= lb(x);
    }
    return ret;
}

LL solve(int n)
{
    sort(node+1,node+n+1,cmp1);
    for(int i=1;i<=n;i++)
        node[i].chg = i;
    sort(node+1,node+n+1,cmp2);
    memset(c,0,sizeof c);
    for(int i=1;i<=n;i++){
        fir[i] = i - 1 - query(node[i].chg - 1);
        update(node[i].chg,1);
    }
    memset(c,0,sizeof c);
    for(int i=n;i>0;i--){
        sec[i] = query(node[i].chg - 1);
        update(node[i].chg,1);
    }
    LL ret = 0;
    for(int i=1;i<=n;i++)
        ret += (LL) fir[i] * sec[i];
    return ret;
}

int main()
{
    int n;
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        scanf("%d",&node[i].init);
        node[i].id = i;
    }
    //printf("%I64d\n",solve(n));
    cout<<solve(n)<<endl;
    return 0;
}