HDU 2095 find your present (2)

#include <iostream>
#include <cstdio>
#include <map>
using namespace std;

map<int, int> NUM;
int main(){
    int n;
    while(1){
        NUM.clear();
        scanf("%d", &n);
        if(n == 0)
            break;

        int num;
        for(int i = 0; i < n; i++){
            scanf("%d", &num);
            NUM[num] += 1;
        }

        map<int, int>::iterator it;
        it = NUM.begin();
        for(; it != NUM.end(); it++){
            int k = it->second;
            if(k % 2){
                cout << it->first << endl;
                break;
            }
        }

    }

    return 0;
}