【模板】逆序对

转载于 2018-12-22 18:56:00 发布 · 83 阅读

CC 4.0 BY-SA版权

原文链接：http://www.cnblogs.com/kcn999/p/10222646.html

int n;
int a[MAX_N];
int b[MAX_N];
long long ans;

void Merge_Sort(int lt, int rt)
{
    if(lt == rt) return;
    int mid = lt + rt >> 1;
    Merge_Sort(lt, mid);
    Merge_Sort(mid + 1, rt);
    int i = lt, j = mid + 1;
    int cnt = 0;
    while(i <= mid && j <= rt)
    {
        if(a[i] <= a[j]) b[++cnt] = a[i++];
        else ans += mid - i + 1, b[++cnt] = a[j++];
    }
    while(i <= mid) b[++cnt] = a[i++];
    while(j <= rt) b[++cnt] = a[j++];
    for(register int i = lt; i <= rt; ++i)
    {
        a[i] = b[i - lt + 1];
    }
    return;
}