蒟蒻の疑惑

#include<iostream>
#include<algorithm>
#include<limits.h>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<vector>
#include<cmath>
#include<queue>
#include<stack>
#include<map>
#define int long long
#define endl '\n'
using namespace std;
struct node{
    int val,pos;
}a[1145141];
int n,ans=0;
int tree[1145141];
int lowbit(int x){
    return x&-x;
}
void add(int x,int k){
    for(;x<=n;x+=lowbit(x)) tree[x]+=k;
}
int ask(int x){
    int ans=0;
    for(;x;x-=lowbit(x)) ans+=tree[x];
    return ans;
}
bool cmp(node x,node y){
    if(x.val!=y.val){
        return x.val<y.val;
    }
    return x.pos<y.pos;
}
signed main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cin>>n;
    memset(tree,0,sizeof(tree));
    for(int i=1;i<=n;i++){
        cin>>a[i].val;
        a[i].pos=i;
    }
    stable_sort(a+1,a+n+1,cmp);
   //--------------------------
    for(int i=n;i>=1;i--){
        ans+=ask(a[i].pos);
        add(a[i].pos,1);
    }
    //--------------------------
    cout<<ans<<endl;
    return 0;
}

看着题解区dalao的代码自己敲了一遍

发现两条分割线内的代码还是不懂qwq

望得到解答，感谢