HDU 2095 find your present (2)

find the special present in the n cards (1<=n<1000000)

#include<cstdio>
#include<set>

using namespace std;

int main()
{
    int n, e;
    set<int> s;
    while (scanf("%d", &n), n)
    {

        while (n--)
        {
            scanf("%d", &e);
            if (s.find(e) == s.end())
                s.insert(e);
            else
                s.erase(e);
        }
        printf("%d\n", *s.begin());
        s.clear();
    }
    return 0;
}

借助异或的性质

int main()
{
    int n, e, ans;
    while (scanf("%d", &n), n)
    {
        //  0^n = n
        //  n^n = 0
        ans = 0;  
        while (n--)
        {
            scanf("%d", &e);
        //  a^b = b^a。
        //  (a^b)^c = a^(b^c)
        //  a^b^a = b
            ans ^= e;  
        }
        printf("%d\n", ans);
    }
    return 0;
}