poj 2299 Ultra-QuickSort

归并排序模版：

 1 #include<stdio.h>

 2 

 3 long long int ans, n;

 4 long long int a[501000], b[501000];

 5 

 6 void mergesort(int start, int end)

 7 {

 8     if(start < end)

 9     {

10         int i, j, mid = (start+end)>>1;

11         mergesort(start, mid);

12         mergesort(mid+1, end);

13         int t = start;

14         j = mid+1; i = start;

15         while(i<=mid && j <= end)

16         {

17             if(a[i] <= a[j])

18                 b[t++] = a[i++];

19             else

20             {

21                 b[t++] = a[j++];

22                 ans += mid-i+1;

23             }

24         }

25         while(i<=mid)

26             b[t++] = a[i++];

27         while(j<=end)

28             b[t++] = a[j++];

29         for(int k = start; k <= end; k ++)

30         a[k] = b[k];

31     }

32 }

33 

34 int main()

35 {

36     while(scanf("%lld",&n))

37     {

38         ans = 0;

39         if(n == 0) break;

40         for(int i = 0; i < n; i ++)

41             scanf("%lld",&a[i]);

42         mergesort(0,n-1);

43         printf("%lld\n",ans);

44     }    

45     return 0;

46 }