使用归并排序求解逆序对

Reverse-Pair(Merge-Sort).cpp

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+#include <cstdio>
+
+#define INF 1000000000
+#define ELEMENT_COUNT 100000
+
+using namespace std;
+
+int n, d[ELEMENT_COUNT], ans;
+
+void merge(int o1, int l1, int o2, int l2)
+{
+	static int a1[ELEMENT_COUNT], a2[ELEMENT_COUNT];
+	int k1 = 0, k2 = 0;
+	for (int i = 0; i < l1; i++)
+	{
+		a1[i] = d[i + o1];
+	}
+	a1[l1] = INF;
+	for (int i = 0; i < l2; i++)
+	{
+		a2[i] = d[i + o2];
+	}
+	a2[l2] = INF;
+	for (int i = 0; i < l1 + l2; i++)
+	{
+		if (a1[k1] <= a2[k2])
+		{
+			d[i + o1] = a1[k1++];
+		}
+		else
+		{
+			d[i + o1] = a2[k2++];
+			ans += (l1 - k1);
+		}
+	}
+}
+
+void merge_sort(int l, int r)
+{
+	if (l < r)
+	{
+		int mid = (l + r) >> 1;
+		merge_sort(l, mid);
+		merge_sort(mid + 1, r);
+		merge(l, mid - l + 1, mid + 1, r - mid);
+	}
+}
+
+int main()
+{
+	scanf("%d", &n);
+	for (int i = 0; i < n; i++)
+	{
+		scanf("%d", &d[i]);
+	}
+	merge_sort(0, n - 1);
+	printf("%d", ans);
+	return 0;
+}