归并排序 & 逆序对

归并排序与逆序对联系紧密，使归并排序成为了必会的排序之一。

void msort(int l,int r){if(l == r) return;int mid = (l + r) >> 1;msort(l,mid);msort(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 <= r) b[k++] = a[j++];for(int i = l; i <= r; i++) a[i] = b[i];}

归并排序时间复杂度O(n log n)