逆数对统计（树状数组 + 离散化）

# include <iostream>
# include <cstdio>
# include <cstring>
# include <algorithm>
using namespace std;
int sum[100002];
int a[100002],b[100002],n;
int lowbit(int k){
    return k & -k;
}
void updata(int k,int data){
    while(k <= n){
        sum[k] += data;
        k += lowbit(k);
    }
}
int Sum(int k){
    int ans = 0;
    while(k){
        ans += sum[k];
        k -= lowbit(k);
    }
    return ans;
}
int main(){
    scanf("%d",&n);
    for(int i = 1;i <= n;i++){
        scanf("%d",&a[i]);
        b[i] = a[i];
    }
    sort(b + 1,b + n + 1);
    int ans = 0;
    for(int i = 1;i <= n;i++){
        int k = lower_bound(b + 1,b + n + 1,a[i]) - b;
        updata(k,1);
        ans += i - Sum(k);
    }
    printf("%d\n",ans);
}