【逆序數】POJ 2188

這題也是求逆序數，只是題目描述有點詭異==！給出的是鉤子的id，不是一對一的關係，所以要映射一下，喜歡上用歸併求逆序數

#define N 1010int a[N],b[N];int tmp[N];int sum;void merge_sort(int l,int mid,int r){    int i = l,j = mid+1,k = 1;    while(i<=mid && j<=r){        if(a[j]<a[i]){            tmp[k++] = a[j++];            sum += mid-i+1;        } else tmp[k++] = a[i++];    }    while(i<=mid)tmp[k++] = a[i++];    while(j<=r)tmp[k++] = a[j++];    for(i=l,k=1;i<=r;i++,k++){        a[i] = tmp[k];    }}void merge(int s,int t){    if(s==t)return ;    int mid = (s+t)>>1;    merge(s,mid);    merge(mid+1,t);    merge_sort(s,mid,t);}int main(){    int n;    while(scanf("%d",&n) != -1){        int i,j;        for(i=1;i<=n;i++){            scanf("%d%d",&a[i],&b[i]);            tmp[a[i]] = i;        }        for(i=1;i<=n;i++){            a[i] = tmp[b[i]];        }        sum = 0;        merge(1,n);        printf("%d\n",sum);    }    return 0;}