merge_sort

void merge_sort(int l,int r) {
    if (l == r) {
        return;
    }
    int mid = l + r >> 1;
    merge_sort(l, mid);
    merge_sort(mid + 1, r);
    int i = l, j = mid + 1, k = l;
    while (i <= mid && j <= r) {
        if (a[i] <= a[j]) {
            b[k++] = a[i++];
        } else {
            ans+=mid-i+1;
            b[k++] = a[j++];
        }
    }
    while (i <= mid) {
        b[k++] = a[i++];
    }
    while (j <= mid) {
        b[k++] = a[j++];
    }
    for (int i = l; i <= r; i++) {
        a[i] = b[i];
    }
}