樹狀數組求逆序數的模板

#include<stdio.h>#include<string.h>#include<algorithm>using namespace std;#define lowbit(x) x&(-x)int res[1000005] , tree[1000005] ;struct num{    int val,pos;}m[1000005];bool cmp (num a, num b){    return a.val<b.val;}void plus ( int x ,int n ){        //樹狀數組儲存法    while( x <= n ){        tree[x] ++;        x += lowbit(x);    }}int SUM ( int x ){               //樹狀數組的部分了    int s = 0;    while(x){        s += tree[x];        x -= lowbit(x) ;    }    return s;}int main(){    int n , x , a , b ;    while( ~scanf( "%d" , &n ) )    {        memset( m , 0 , sizeof(m) );        memset( res , 0 , sizeof(res) );        memset( tree , 0 , sizeof(tree) );        for( int i = 1 ; i <= n ; i++ ){            scanf( "%d" , &x );            m[i].val = x ;            m[i].pos = i ;        }        sort( m+1 , m+n+1 , cmp );        for( int i = 1 ; i <= n ; i++ ){     //離散化            res[m[i].pos] = i ;        }        __int64 sum = 0;        for( int i = 1 ; i <= n ; i++ ){            plus( res[i] , n );            sum += ( i - SUM( res[i]-1 ) - 1) ;        }        printf("%I64d\n",sum);    }    return 0;}