hdu 1.2.4

采用异或...

 #include<stdio.h>
 int main()
 {
     //freopen("input.txt","r",stdin);
     int n,s,a;
     while(scanf("%d",&n),n)
     {
         s = ;
         while(n--)
         {
             scanf("%d",&a);
             s ^= a;
         }
         printf("%d\n",s);
     }
 
     return ;
 }