P1494 [国家集训队] 小 Z 的袜子

https://www.luogu.com.cn/record/76778088

Luogu/P1494/P1494.cpp

@@ -0,0 +1,77 @@
+#include <iostream>
+#include <algorithm>
+#include <cmath>
+#include <tuple>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 50005;
+
+int n, m, a[N], c[N], t;
+std::pair<long long, long long> now, ans[N];
+std::vector<std::tuple<int, int, int>> q;
+
+void add(int x) {
+    if (++c[x] > 1) {
+        now.first += c[x] * (c[x] - 1) - (c[x] - 1) * (c[x] - 2);
+    }
+}
+
+void del(int x) {
+    if (--c[x]) {
+        now.first += c[x] * (c[x] - 1) - (c[x] + 1) * c[x];
+    }
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n >> m;
+
+    t = sqrt(n);
+
+    for (int i = 1; i <= n; i++) {
+        cin >> a[i];
+    }
+
+    for (int i = 1, l, r; i <= m; i++) {
+        cin >> l >> r;
+
+        q.push_back(std::make_tuple(i, l, r));
+    }
+
+    std::sort(q.begin(), q.end(), [&](auto a, auto b) {
+        int p = std::get<1>(a) / t,
+            q = std::get<1>(b) / t;
+
+        return p == q ? std::get<2>(a) < std::get<2>(b) : p < q;
+    });
+
+    int i = 0, j = 1, x, l, r;
+    for (auto e : q) {
+        std::tie(x, l, r) = e;
+
+        if (l == r) {
+            ans[x] = std::make_pair(0, 1);
+        } else {
+            while (i < r) add(a[++i]);
+            while (i > r) del(a[i--]);
+            while (j < l) del(a[j++]);
+            while (j > l) add(a[--j]);
+
+            now.second = 1ll * (r - l + 1) * (r - l);
+            int g = std::__gcd(now.first, now.second);
+            ans[x] = std::make_pair(now.first / g, now.second / g);
+        }
+    }
+
+    for (int i = 1; i <= m; i++) {
+        cout << ans[i].first << '/' << ans[i].second << endl;
+    }
+
+    return 0;
+}