P1908 逆序对

R43026624

problem/P1908/P1908.cpp

@@ -0,0 +1,41 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+
+long long n, ans, a[500005], t[500005];
+
+void merge_sort(long long a[], long long l, long long r) {
+    if (l >= r) return;
+    int mid = l + r >> 1;
+    merge_sort(a, l, mid);
+    merge_sort(a, mid + 1, r);
+    int i = l, j = mid + 1, k = 0;
+    while (i <= mid && j <= r) {
+        if (a[i] <= a[j]) {
+            t[k++] = a[i++];
+        }
+        else {
+            t[k++] = a[j++];
+            ans += (mid - i + 1);
+        }
+    }
+    while (i <= mid) {
+        t[k++] = a[i++];
+    }
+    while (j <= r) {
+        t[k++] = a[j++];
+    }
+    for (int i = l, j = 0; i <= r; i++, j++) {
+        a[i] = t[j];
+    }
+}
+
+int main() {
+    cin >> n;
+    for (int i = 1; i <= n; i++) {
+        cin >> a[i];
+    }
+    merge_sort(a, 1, n);
+    cout << ans << endl;
+    return 0;
+}