2014 HDU多校弟五场A题 【归并排序求逆序对】

 1 #include <stdio.h>
 2 #include <string.h>
 3 #include <stdlib.h>
 4 #include <math.h>
 5 #include <iostream>
 6 #include <stack>
 7 #include <queue>
 8 #include <algorithm>
 9 
10 #define ll long long
11 using namespace std;
12 const int INF = 0x3f3f3f3f;
13 const int MAXN = 1111;
14 const int N = 100005;
15 
16 int a[N],tmp[N];
17 //int array_a[N], array_b[N];
18 ll ans;
19 int n, k;
20 
21 void Merge(int l,int m,int r)
22 {
23     int i = l;
24     int j = m + 1;
25     int k = l;
26     while(i <= m && j <= r)
27     {
28         if(a[i] > a[j])
29         {
30             tmp[k++] = a[j++];
31             ans += m - i + 1;
32         }
33         else
34         {
35             tmp[k++] = a[i++];
36         }
37     }
38     while(i <= m) tmp[k++] = a[i++];
39     while(j <= r) tmp[k++] = a[j++];
40     for(int i=l;i<=r;i++)
41         a[i] = tmp[i];
42 }
43 
44 void Merge_sort(int l,int r)
45 {
46     if(l < r)
47     {
48         int m = (l + r) >> 1;
49         Merge_sort(l,m);
50         Merge_sort(m+1,r);
51         Merge(l,m,r);
52     }
53 }
54 
55 int main()
56 {
57     int i, j;
58     while(EOF != scanf("%d%d",&n,&k)){
59         for(i=0;i<n;i++){
60             scanf("%d",&a[i]);
61             //array_a[i] = array_b[i] = a[i];
62         }
63         ans = 0;
64         Merge_sort(0,n-1);
65         ans -= k;
66         if(ans <= 0 )    printf("0\n");
67         else cout << ans << endl;
68     }
69     return 0;
70 }