'commit'

TangDou/AcWing_TiGao/T6/PaiXu/107.cpp

@@ -0,0 +1,38 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+typedef long long LL;
+const int N = 500010;
+int n;
+LL q[N], t[N];
+LL ans; // 结果
+
+void merge_sort(LL 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])
+            t[k++] = q[i++];
+        else {
+            ans += mid - i + 1; // 如果q[i]>q[j],那么 i~mid之间所有数字，都>q[j]
+            t[k++] = q[j++];
+        }
+    // 清理剩余
+    while (i <= mid) t[k++] = q[i++];
+    while (j <= r) t[k++] = q[j++];
+    // 抄回来
+    for (i = l, j = 0; i <= r; i++, j++) q[i] = t[j];
+}
+
+int main() {
+    while (cin >> n, n) {
+        for (int i = 1; i <= n; i++) cin >> q[i];
+        ans = 0;
+        merge_sort(q, 1, n);
+        printf("%lld\n", ans);
+    }
+    return 0;
+}