codevs1688 求逆序对

#include<iostream>
#include<cstdio>
#include<string>
#include<cstring>
#include<algorithm>
#include<vector>
#include<queue>
using namespace std;
const int maxn = 100050;
int n,num[maxn+50],c[maxn+50];
long long sum;
int lowbit(int x){
    return x&(-x);
}
void change(int p,int x){
    for(int i = p;i <= maxn;i+=lowbit(i)) c[i] += x;
}
int query(int a,int b){
    int sum = 0;
    for(int i = b;i;i-=lowbit(i)) sum += c[i];
    for(int i = a-1;i;i-=lowbit(i)) sum -= c[i];
    return sum;
}
int main(){
    cin>>n;
    for(int i = 1;i <= n;i++){
        scanf("%d",&num[i]);
        num[i]++;
        change(num[i],1);
        sum += i-query(1,num[i]);
    }
    cout<<sum;
    return 0;
}