【题解】洛谷P3660 [USACO17FEB]Why Did the Cow Cross the Road III

题目地址

又是一道奶牛题

从左到右扫描，树状数组维护【左端点出现而右端点未出现】的数字的个数。记录每个数字第一次出现的位置。

若是第二次出现，那么删除第一次的影响。

#include <cstdio>
#include <cstring>
#define re register
#define GC getchar()
#define Lowbit(X) (X&(-X))
#define Clean(X,K) memset(X,K,sizeof(X))
int Qread () {
    int X = 0 ;    char C = GC ;
    while (C > '9' || C < '0') C = GC ;
    while (C >='0' && C <='9') {X = X * 10 + C - '0';C = GC ;}
    return X ;
}
const int Maxn = 50005 << 1 ;
int N , Vis[Maxn >> 1] , A[Maxn] , Head[Maxn >> 1] , T[Maxn];
unsigned long long int Ans = 0 ;
int Ask (int X) {re int Ans = 0 ;while (X > 0) {    Ans += T[X] ;    X -= Lowbit(X) ;}return Ans ;}
void Add (int X , int K) {while (X <= N) {T[X] += K;X += Lowbit(X) ;}}
int main () {
    N = Qread () * 2 , Clean(Vis , 0) ;
    for (re int i = 1 ;i <= N; ++ i) A[i] = Qread () ;
    for (re int i = 1 ;i <= N; ++ i)if (Vis[A[i]]) Add (Head[A[i]] , -1) ,    Ans += Ask (i) - Ask (Head[A[i]]) ;
        else Head[A[i]] = i , Add (i , 1) , Vis[A[i]] = 1 ;
    printf ("%lld
" , Ans) ;
    fclose (stdin) ,fclose (stdout);
    return 0;
}