POJ2299 [Ultra-QuickSort] 值域树状数组

题解：值域树状数组

值域树状数组就是以值域建树。比如，一个数是 x ， 那么我们建树的方法就是在x的地方加1.那么如何统计逆序对的个数呢？我们的方法就是插入每个数后，看在这个数之前插入的数中，有多少个数比它大，然后加入答案。

# define Name ""# include <cstdio># include <cstring># include <iostream># include <algorithm># define LL long longusing namespace std ;const int N = 5e5 + 10 ;int a [N] , b [N] , c [N] , n ;int lowbit ( int x ) {    return ( - x ) & x ;}void modify ( int pos , int val ) {    for ( int i = pos ; i <= n ; i += lowbit ( i ) )         c [i] += val ;}int query ( int pos ) {    int ret = 0 ;    for ( int i = pos ; i >= 1 ; i -= lowbit ( i ) )         ret += c [i] ;    return ret ;}int main () {    while ( scanf ( "%d" , & n ) != EOF && n ) {        memset ( c , 0 , sizeof c ) ;        for ( int i = 1 ; i <= n ; ++ i ) scanf ( "%d" , & a [i] ) , b [i] = a [i] ;        sort ( a + 1 , a + 1 + n ) ;        for ( int i = 1 ; i <= n ; ++ i ) b [i] = lower_bound ( a + 1 , a + 1 + n , b [i] ) - a ;        LL ans = 0 ;        for ( int i = 1 ; i <= n ; ++ i ) {            modify ( b [i] , 1 ) ;             ans += i - query ( b [i] ) ;        }        printf ( "%lld\n" , ans ) ;     }    return 0 ;}