题目地址

Being unique is so important to people on Mars that even their lottery is designed in a unique way. The rule of winning is simple: one bets on a number chosen from [1,10
​4
​​ ]. The first one who bets on a unique number wins. For example, if there are 7 people betting on { 5 31 5 88 67 88 17 }, then the second one who bets on 31 wins.

Input Specification:
Each input file contains one test case. Each case contains a line which begins with a positive integer N (≤10
​5
​​ ) and then followed by N bets. The numbers are separated by a space.

Output Specification:
For each test case, print the winning number in a line. If there is no winner, print None instead.

Sample Input 1:
7 5 31 5 88 67 88 17

Sample Output 1:
31

Sample Input 2:
5 888 666 666 888 888

Sample Output 2:
None

#include <iostream>
#include <vector>
#include<algorithm>
#include <cmath>
#include<map>
#include<cstring>
#include<queue>
#include<string>
#include<set>
using namespace std;
typedef long long ll;
const int maxn=100010,inf=100000000;
map<int,int>exsit;
int main(){
    int n,a[maxn];
    cin>>n;
    for(int i=0;i<n;i++){
         cin>>a[i];exsit[a[i]]++;
    }
    for(int i=0;i<n;i++){
        if(exsit[a[i]]==1){
            cout<<a[i];return 0;
        }
    }
    cout<<"None";
}