2 Yarik and Musical Notes

打表找到的规律

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=2e5+10;
int a[N];
void solve(){
	int n;
	cin>>n;
	map<int,int>mp;
	for(int i=1;i<=n;i++){
		cin>>a[i];
		mp[a[i]]++;
	}
	int ans=0;
	for(int i=n;i>=1;i--){
		if(a[i]==2){
			ans+=mp[1];
			ans+=mp[2]-1;
			mp[2]--;
		}else if(a[i]==1){
			ans+=mp[1]-1;
			ans+=mp[2];
			mp[1]--;
		}else{
			ans+=mp[a[i]]-1;
			mp[a[i]]--;
		}
	}
	cout<<ans<<"\n"; 
}
signed main(){
	ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
	int t=1;
	cin>>t;
	for(int i=1;i<=t;i++)solve();
	return 0;
}