Inversions

Input

Output

Sample Input

1. #include"iostream"
2. #include"algorithm"
3. #include"cstring"
4. #include"cstdio"
5. using namespace std;
6. #define max 65550
7. structab
8. {
9. int value;
10. int index;
11. }a[max];
12. /*bool cmp(const ab &a,const ab &b)
13. {
14. if(a.value!=b.value)
15. return a.value<b.value;
16. else
17. return a.index<b.index;
18. }*/
19. bool cmp(const ab&a,const ab&b)
20. {
21.     return a.value<b.value;
22. }
23. int c[max],b[max];
24. int n;
25. int lowbit(int x)
26. {
27. return x&(-x);
28. }
29. void updata(int x,int d)
30. {
31. while(x<=n)
32. {
33. c[x]+=d;
34. x+=lowbit(x);
35. }
36. }
37. int getsum(int x)
38. {
39. int res=0;
40. while(x>0)
41. {
42. res+=c[x];
43. x-=lowbit(x);
44. }
45. return res;
46. }
47. int main()
48. {
49. int i;
50. scanf("%d",&n);
51. for(i=1;i<=n;i++)
52. {
53. scanf("%d",&a[i].value);
54. a[i].index=i;
55. }
56. sort(a+1,a+1+n,cmp);
57. b[a[1].index]=1;
58. memset(c,0,sizeof(c));
59. //memset(b,0,sizeof(b));
60. for(i=2;i<=n;i++)
61. {
62. if(a[i].value!=a[i-1].value)
63. b[a[i].index]=i;
64. else
65.          b[a[i].index]=b[a[i-1].index];
66. }
67. long long int sum=0;
68. for(i=1;i<=n;i++)
69. {
70. updata(b[i],1);
71. sum+=getsum(n)-getsum(b[i]);
72. }
73. printf("%lld\n",sum);
74. return 0;
75. }

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Inversions

原文：http://www.cnblogs.com/767355675hutaishi/p/3683429.html