HDU 4944 逆序数对

 1 #include <bits/stdc++.h>
 2 
 3 using namespace std;
 4 
 5 const int maxn = 1e5 + 5;
 6 
 7 __int64 cnt,k;
 8 int a[maxn],c[maxn];
 9 
10 
11 void merge(int* a,int first,int mid,int last,int* c) {
12     int i = first,j=mid+1;
13     int m = mid,n=last;
14     int k = 0;
15     while(i<=m||j<=n) {
16         if(j>n||(i<=m&&a[i]<=a[j])) {
17             c[k++] = a[i++];
18         }
19         else {
20             c[k++] = a[j++];
21             cnt += (m-i+1);
22         }
23     }
24     for(i=0;i<k;i++)
25         a[first+i] = c[i];
26 }
27 
28 void mergeSort(int* a,int first,int last,int* c) {
29     if(first<last) {
30         int mid = (first+last)/2;
31         mergeSort(a,first,mid,c);
32         mergeSort(a,mid+1,last,c);
33         merge(a,first,mid,last,c);
34     }
35 }
36 
37 int main()
38 {
39     int n;
40     while(~scanf("%d%I64d",&n,&k)) {
41         for(int i=0;i<n;i++)
42             scanf("%d",&a[i]);
43         memset(c,0,sizeof(c));
44         cnt = 0;
45         mergeSort(a,0,n-1,c);
46         printf("%lld\n",max(cnt-k,0LL));
47     }
48     return 0;
49 }