方法一：樹狀數組

模板如下：

#include<iostream>
#include<cstdio>
#include <algorithm>
using namespace std;
const int MAXN=40001;
int n, a[MAXN], tree[MAXN], tot;

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


int query(int x) {
    int ans=0;
    while (x>0) {
        ans+=tree[x];
        x-=lowbit(x);
    }
    
 
return ans;
}

void addz(int x) {
    while (x<=n) {
        tree[x]+=1;
        x+=lowbit(x);
    }
    return;
}

struct Node {
    int v, ord;
    bool operator < (const Node x) const { return v<x.v; }
}node[MAXN];

int main() {
    scanf("%d", &n);
    for (int i=1; i<=n; i++) {
        scanf(
 
"%d", &node[i].v);
        node[i].ord=i;
    }
    sort(node+1, node+n+1 );
    for (int i=1; i<=n; i++) a[node[i].ord]=i;
    for (int i=1; i<=n; i++) {
        addz(a[i]);
        tot+=i-query(a[i]); 
    }
    printf("%d", tot);
  return 0;
}

求逆序對