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 ", res); 41 } 42 return 0; 43 }