下列说法正确的是

void merge_sort(int l , int r) {

if(l>=r) return;

int mid = (l+r) / 2 ;

merge_sort(l , mid);

merge_sort(mid+1 , r);

int i = l;

int p = l , q = mid+1;

while(p<=mid || q<=r) {

if(q>r || (p<=mid && a[p]<=a[q]))

b[i++] = a[p++];

else {

b[i++] = a[q++];

cnt += mid -p + 1;

}

}

for(i = l ; i <= r; i++)

a[i] = b[i];}