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 }
posted on 2012-05-14 22:51 Try86 阅读(156) 评论(0) 收藏举报