#137. 【2020.11.28 NOIP模拟赛 T2】交换

https://sjzezoj.com/submission/70885

S2OJ/137/137.cpp

@@ -0,0 +1,49 @@
+#include <iostream>
+#include <numeric>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 3e5 + 5;
+
+int n, a[N], c1[N], c2[N], ans[N];
+
+int lowbit(int x) {
+    return x & -x;
+}
+
+void add(int* c, int x, int y) {
+    for (; x <= n; x += lowbit(x)) c[x] += y;
+}
+
+int sum(int* c, int x) {
+    int res = 0;
+    for (; x; x -= lowbit(x)) res += c[x];
+    return res;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n;
+
+    for (int i = 1; i <= n; i++) {
+        cin >> a[i];
+    }
+
+    for (int i = 1; i <= n; i++) {
+        ans[i] = sum(c1, n) - sum(c1, a[i]);
+        add(c1, a[i], 1);
+    }
+
+    for (int i = n; i; i--) {
+        ans[i] = std::min(ans[i], sum(c2, n) - sum(c2, a[i]));
+        add(c2, a[i], 1);
+    }
+
+    cout << std::accumulate(ans + 1, ans + 1 + n, 0ll) << endl;
+
+    return 0;
+}