#639. 串一串（string）

https://sjzezoj.com/submission/71323

S2OJ/639/639.cpp

@@ -0,0 +1,72 @@
+#include <iostream>
+#include <algorithm>
+#include <string>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 1e6 + 5,
+          M = 62;
+const int mod = 998244353;
+
+int n, a[N], pre[N][M], suf[N][M], f1[M][M], f2[M][M], same, ans;
+std::string s;
+
+int c2(int x) {
+    return static_cast<long long>(x) * (x - 1) / 2 % mod;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> s;
+
+    n = s.size();
+    std::transform(s.rbegin(), s.rend(), a + 1, [&](char c) -> int {
+        if ('a' <= c && c <= 'z') return c - 'a';
+        if ('A' <= c && c <= 'Z') return c - 'A' + 26;
+        return c - '0' + 52;
+    });
+
+    for (int i = 1; i <= n; i++) {
+        std::copy(std::begin(pre[i - 1]), std::end(pre[i - 1]), std::begin(pre[i]));
+        pre[i][a[i]]++;
+    }
+
+    for (int i = n; i; i--) {
+        std::copy(std::begin(suf[i + 1]), std::end(suf[i + 1]), std::begin(suf[i]));
+        suf[i][a[i]]++;
+    }
+
+    for (int j = 0; j < M; ++j) {
+        same = (same + c2(pre[n][j])) % mod;
+    }
+
+    for (int i = 1; i <= n; i++) {
+        same = ((same - suf[i + 1][a[i]]) % mod + mod) % mod;
+
+        for (int j = 0; j < M; j++) {
+            if (j == a[i]) continue;
+
+            ans = (ans + static_cast<long long>(f2[j][a[i]]) * ((c2(n - i - suf[i + 1][a[i]] - suf[i + 1][j]) - ((same - (c2(suf[i + 1][j]) + c2(suf[i + 1][a[i]]))) % mod + mod) % mod) % mod + mod) % mod) % mod;
+        }
+
+        for (int j = 0; j < M; j++) {
+            if (j == a[i]) continue;
+
+            f2[a[i]][j] = (f2[a[i]][j] + f1[a[i]][j]) % mod;
+        }
+
+        for (int j = 0; j < M; j++) {
+            if (j == a[i]) continue;
+
+            f1[j][a[i]] = (f1[j][a[i]] + pre[i - 1][j]) % mod;
+        }
+    }
+
+    cout << ans << endl;
+
+    return 0;
+}