萌新汉子刚学OI，救救孩子吧

树状数组

开了longlong

用了读入优化

TLE

50pts

#include<cstdio>
#include<iostream>
#include<map>
#include<algorithm>
#define int long long
using namespace std;
typedef long long ll;

inline void input(ll &x){
    ll ans=0,f=1;
    char c=getchar();
    while(c>'9'||c<'0'){
        if(c=='-')f=-1;
        c=getchar();
    }
    while(c>='0'&&c<='9'){
        ans=ans*10+c-48;
        c=getchar();
    }
    x=ans*f;
}

inline void output(ll x){
    if(x<0)x=-x,putchar('-');
    if(x>9)output(x/10);
    putchar(x%10+48);
}

inline void writeln(ll x){
    output(x);
    putchar('\n');
}

int a[5000005],b[5000005],c[20000005],n,cnt;

map<int,int> sign;

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

inline void add(int k){
    while(k<=n){
        c[k]++;
        k+=lowbit(k);
    }
}

inline int query(int k){
    int ans=0;
    while(k){
        ans+=c[k];
        k-=lowbit(k);
    }
    return ans;
}

signed main(){
    input(n);
    for(int i=1;i<=n;i++){
        input(a[i]);b[i]=a[i];
    }
    sort(b+1,b+n+1);
    for(int i=1;i<=n;i++){
        sign[b[i]]=++cnt;
    }
    int ans=0;
    for(int i=n;i>=1;i--){
        ans+=query(sign[a[i]]-1);
        add(sign[a[i]]);
    }
    writeln(ans);
}