九度oj 1348 数组中的逆序对

原题链接：http://ac.jobdu.com/problem.php?pid=1348 
归并排序求逆序对。。。

 1 #include<algorithm>

 2 #include<iostream>

 3 #include<cstdlib>

 4 #include<cstring>

 5 #include<cstdio>

 6 typedef unsigned long long ull;

 7 const int Max_N = 100010;

 8 ull res;

 9 int arr[Max_N], temp[Max_N];

10 void Merge(int *A, int l, int m, int r) {

11     int p = 0;

12     int x = l, y = m + 1;

13     while (x <= m && y <= r) {

14         if (A[x] > A[y]) res += m - x + 1, temp[p++] = A[y++];

15         else temp[p++] = A[x++];

16     }

17     while (x <= m) temp[p++] = A[x++];

18     while (y <= r) temp[p++] = A[y++];

19     for (x = 0; x < p; x++) A[l + x] = temp[x];

20 }

21 void MergeSort(int *A, int l, int r) {

22     int mid;

23     if (l < r) {

24         mid = (l + r) >> 1;

25         MergeSort(A, l, mid);

26         MergeSort(A, mid + 1, r);

27         Merge(A, l, mid, r);

28     }

29 }

30 int main() {

31 #ifdef LOCAL

32     freopen("in.txt", "r", stdin);

33     freopen("out.txt", "w+", stdout);

34 #endif

35     int n;

36     while (~scanf("%d", &n)) {

37         res = 0;

38         for (int i = 0; i < n; i++) scanf("%d", &arr[i]);

39         MergeSort(arr, 0, n - 1);

40         printf("%lld\n", res);

41     }

42     return 0;

43 }

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