hdu 1394 inversion number ++ARRAY TREE

求逆序数

#include <stdio.h>#include <string.h>#define N 5050int a[N],n;int lowbit(int i){    return i&(-i);}void add(int i){    for(;i<=n; i += lowbit(i))        a[i] ++;}int sum(int i){    int s = 0;    for(; i > 0; i -= lowbit(i))        s += a[i];    return s;}int main(){    while(scanf("%d",&n)!= EOF)    {        int i,res = 0,b[N];        memset(a,0,sizeof(a));        for(i = 0; i < n; ++i)        {            scanf("%d",&b[i]);            ++b[i];            res += sum(n) - sum(b[i]);            add(b[i]);        }        int tmp = res;        ///新的逆序数只由移动的最左端的数决定        for(i = 0; i < n; ++i)        {            tmp = tmp + sum(n) - sum(b[i]) -sum(b[i] - 1);            if(tmp < res)                res = tmp;        }        printf("%d\n",res);    }    return 0;}