788. 逆序对的数量
#include <iostream> using namespace std; const int N = 1e6 + 10; int n; int q[N], tmp[N]; long long cnt; void merge_sort(int q[], int l, int r) { if (l >= r) return ; int mid = (l + r) >> 1; merge_sort(q, l, mid); merge_sort(q, mid + 1, r); int i = l, j = mid + 1, k = 0; while (i <= mid && j <= r) { if (q[i] <= q[j]) tmp[k ++] = q[i ++]; else { cnt += mid - i + 1; tmp[k ++] = q[j ++]; } } while (i <= mid) tmp[k ++] = q[i ++]; while (j <= r) tmp[k ++] = q[j ++]; for (int i = l, j = 0; i <= r; ++ i, ++ j) q[i] = tmp[j]; return ; } int main() { cin >> n; for (int i = 0; i < n; ++ i) cin >> q[i]; merge_sort(q, 0, n - 1); cout << cnt << endl; return 0; }