Find the Lost Sock

Alice bought a lot of pairs of socks yesterday. But when she went home, she found that she has lost one of them. Each sock has a name which contains exactly 7 charaters.

Alice wants to know which sock she has lost. Maybe you can help her.

There are multiple cases. The first line containing an integer n (1 <= n <= 1000000) indicates that Alice bought n pairs of socks. For the following 2*n-1 lines, each line is a string with 7 charaters indicating the name of the socks that Alice took back.

The name of the lost sock.

Because of HUGE input, scanf is recommended.

解题思路：

数据量很大，用map存会超时。由于一位学长的指点，了解到了按位异或，当一个数出现了两次的时候就会抵消，所以我们只要不断按位异或，最后剩下的就是答案。

比如 1 1 3 2 2

我们只要按位异或 1->0->3->1->3

最后的3便是所求的答案。

#include <bits/stdc++.h>
using namespace std;
int main()
{
    int n,m,i,j,k;
    while(cin>>n)
    {
        getchar();
        int s[8]={0};char a[10];
        for(i=1;i<=2*n-1;i++)
        {
            gets(a);
            for(j=0;j<7;j++) s[j]=s[j]^(char)a[j];
        }
        for(i=0;i<7;i++) cout<<(char)s[i];
        cout<<"\n";
    }
}