RQNOJ 173(归并排序求逆序数)

 1 /*

 2 *  归并排序求逆序对 

 3 */

 4 

 5 #include <cstdio>

 6 #include <iostream>

 7 

 8 using namespace std;

 9 

10 const int N = 20005;

11 

12 long long ans;

13 int a[N], b[N];

14 

15 void mergeSort(int left, int right) {

16     int i, j, k, mid;

17     if (left < right) {

18         mid = (left + right) >> 1;

19         mergeSort(left, mid);

20         mergeSort(mid+1, right);

21         k = left;

22         for (i=left,j=mid+1; i<=mid&&j<=right;) {

23             if (a[i] > a[j]) b[k++] = a[j++], ans += mid-i+1;//逆序对 

24             else b[k++] = a[i++];

25         }

26         while (i <= mid) b[k++] = a[i++];

27         while (j <= right) b[k++] = a[j++];

28         for (i=left; i<=right; ++i) a[i] = b[i];

29     }

30 }

31 

32 int main() {

33     int n;

34     while (scanf("%d", &n) != EOF) {

35         for (int i=1; i<=n; ++i) scanf ("%d", &a[i]);

36         ans = 0;

37         mergeSort(1, n);

38         printf ("%d\n", ans);

39     }

40     return 0;

41 }