merge_sort

 1 void merge_sort(int l,int r) {
 2     if (l == r) {
 3         return;
 4     }
 5     int mid = l + r >> 1;
 6     merge_sort(l, mid);
 7     merge_sort(mid + 1, r);
 8     int i = l, j = mid + 1, k = l;
 9     while (i <= mid && j <= r) {
10         if (a[i] <= a[j]) {
11             b[k++] = a[i++];
12         } else {
13             ans+=mid-i+1;
14             b[k++] = a[j++];
15         }
16     }
17     while (i <= mid) {
18         b[k++] = a[i++];
19     }
20     while (j <= mid) {
21         b[k++] = a[j++];
22     }
23     for (int i = l; i <= r; i++) {
24         a[i] = b[i];
25     }
26 }