https://www.acmicpc.net/problem/10090

Inversion Counting 기본 문제

코드

#include <bits/stdc++.h>
#define FASTIO ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
using namespace std;
typedef long long ll;

class Seg
{
    private:
        int n;
        vector<int> tree;

        void upd(int node, int start, int end, int idx, int val)
        {
            if(idx<start || end<idx) return;
            if(start == end){
                tree[node]++;
                return;
            }

            int mid = start+end>>1;
            upd(2*node, start, mid, idx, val);
            upd(2*node+1, mid+1, end, idx, val);

            tree[node] = tree[2*node] + tree[2*node+1];
        }

        int qry(int node, int start, int end, int left, int right)
        {
            if(end<left || right<start) return 0;
            if(left<=start && end<=right) return tree[node];

            int mid = start+end>>1;
            int l = qry(2*node, start, mid, left, right);
            int r = qry(2*node+1, mid+1, end, left, right);

            return l+r;
        }

    public:
        void init(int n)
        {
            this->n = n;
            tree.resize(4*n);
        }

        void upd(int idx, int val) {upd(1, 1, n, idx, val);}
        int qry(int left, int right) {return qry(1, 1, n, left, right);}
    
};

signed main()
{
    FASTIO;
    int n;
    cin >> n;
    Seg seg;
    seg.init(n);

    ll ans = 0ll;
    for(int i = 0; i<n; i++){
        int a; cin >> a;
        ans += seg.qry(a+1, n);
        seg.upd(a, 1);
    }

    cout << ans;
    
    return 0;
}