• Source 
    1. /**************************************************************
    2.     Problem: 1106
    3.     User: zrts
    4.     Language: C++
    5.     Result: Accepted
    6.     Time:256 ms
    7.     Memory:1588 kb
    8. ****************************************************************/
    9.  
    10. #include<cstdio>
    11. #include<cstring>
    12. #include<algorithm>
    13. #define lowbit(x) (x&-x)
    14. //by zrt
    15. //problem:
    16. using namespace std;
    17. typedef long long ll;
    18. const double eps(1e-10);
    19. int n;
    20. int c[100005];
    21. void add(int pos,int k){
    22.     for(;pos<=n;pos+=lowbit(pos)){
    23.         c[pos]+=k;
    24.     }
    25. }
    26. int ask(int pos){
    27.     int ret=0;
    28.     for(;pos;pos-=lowbit(pos)){
    29.         ret+=c[pos];
    30.     }
    31.     return ret;
    32. }
    33. int last[100005];
    34. int main(){
    35.     #ifdef LOCAL
    36.     freopen("in.txt","r",stdin);
    37.     freopen("out.txt","w",stdout);
    38.     #endif
    39.     scanf("%d",&n);
    40.     n<<=1;
    41.     int ans=0;
    42.     for(int i=1,k;i<=n;i++){
    43.         scanf("%d",&k);
    44.         if(!last[k]) add(i,1),last[k]=i;
    45.         else ans+=ask(i)-ask(last[k]),add(last[k],-1);
    46.     }
    47.     printf("%d\n",ans);
    48.     return 0;
    49. }