Inverse Pair

​

---

#include<bits/stdc++.h>
using namespace std;
#define int long long
#define IOS ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
int n,m;
int x[200005];
int t[400005];
set<int> s;
//归并排序求逆序数
void merge_sort_inversion(int *A,int l,int r,int *T,int *cnt){
    if (r-l>1) {
        int m=l+(r-l)/2; //划分
        int p=l,q=m,i=l;
        merge_sort_inversion(A, l, m, T,cnt); //递归求解
        merge_sort_inversion(A, m, r, T,cnt); //递归求解
        while (p<m||q<r)
            if (q>=r||(p<m&&A[p]<=A[q]))
                T[i++]=A[p++]; //从左半数组复制到临时空间
            else{
                T[i++]=A[q++]; //从右半数组复制到临时空间
                *cnt += m-p;
            }
        for (i=l; i<r; i++)
            A[i]=T[i]; //从辅助空间复制回A数组
    }
}

void solve(){
    cin>>n;
    for(int i = 1;i <= n;++i)
        cin>>x[i];
    for(int i = 1;i <= n;++i){
        if(s.count(x[i] + 1) != 0)
            ++x[i];
        s.insert(x[i]);
    }
    int ans = 0;
    merge_sort_inversion(x,1,n + 1,t,&ans);
    cout<<ans;
}
signed main(){
    IOS;
    int tt = 1;
    //cin>>tt;
    while(tt--)
        solve();
    return 0;
}