逆序数

归并排序：

#include<bits/stdc++.h>
using namespace std;
const int maxn = 100010;
int n,a[maxn],cnt,temp[maxn],ans;
void Merge_sort(int l,int r)
{
    if(l==r) return ;
    int mid = (l+r)>>1;
    Merge_sort(l,mid);
    Merge_sort(mid+1,r);
    int i = l,j = mid+1,k=l;
    while(i<=mid&&j<=r) {
        if(a[i]>a[j]) {
            ans+=mid-i+1;
            temp[k++] = a[j++];
        }
        else temp[k++] = a[i++];
    }
    while(i<=mid) temp[k++] = a[i++];
    while(j<=r) temp[k++] = a[j++];
    for(int i=l;i<=r;i++) a[i] = temp[i];
}
int main()
{
    std::ios::sync_with_stdio(false);std::cin.tie(0);std::cout.tie(0);
    cin>>n;
    for(int i=0;i<n;i++) cin>>a[i];
    ans=0;
    Merge_sort(0,n-1);
    cout<<ans<<endl;
    return 0;
}

离散化+树状数组

#include<bits/stdc++.h>
using namespace std;
const int maxn = 100010;
int c[maxn],n,a[maxn];
struct node
{
    int order,v;
}q[maxn];
bool cmp(node x,node y) {return x.v<y.v;}
void add(int x,int v) {for(int i=x;i<=n;i+=(i&(-i)))c[i]+=v;}
int query(int x) {int ret=0;for(int i=x;i>0;i-=(i&(-i)))ret+=c[i];return ret;}
int main()
{
    std::ios::sync_with_stdio(false);std::cin.tie(0);std::cout.tie(0);
    int ans=0;
    cin>>n;
    for(int i=1;i<=n;i++){
        cin>>q[i].v;
        q[i].order = i;
    }
    sort(q+1,q+n+1,cmp);
    for(int i=1;i<=n;i++){
        a[q[i].order] = i;
    }
    for(int i=1;i<=n;i++){
        add(a[i],1);
        ans+=i-query(a[i]);
    }
    cout<<ans<<endl;
    return 0;
}