https://pintia.cn/problem-sets/994805342720868352/problems/994805444361437184

#include<bits/stdc++.h>
using namespace std;
const int N=1e5+10;
int a[N],n;
map<int,int>mp;
int main(void)
{
	cin>>n;
	for(int i=0;i<n;i++) cin>>a[i],mp[a[i]]++;
	for(int i=0;i<n;i++)
	{
		if(mp[a[i]]==1) 
		{
			cout<<a[i];
			return 0;
		}
	}
	cout<<"None";
	return 0;
}