#1731. 【2022.11.02 联考】字符串生成（monkey）

https://sjzezoj.com/submission/63219

S2OJ/1731/1731.cpp

@@ -0,0 +1,55 @@
+#include <iostream>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 1e6 + 5;
+const int mod = 1e9 + 7;
+
+int nxt[N], f[N][2], p[N], a[N], b[N];
+std::string s;
+
+int binpow(int a, int b) {
+    int res = 1;
+
+    while (b) {
+        if (b & 1) res = static_cast<long long>(res) * a % mod;
+        a = static_cast<long long>(a) * a % mod;
+        b >>= 1;
+    }
+
+    return res;
+}
+
+signed main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> s;
+
+    s = ' ' + s;
+
+    for (int i = 2, j = 0; i < s.size(); i++) {
+        while (j && s[i] != s[j + 1]) j = nxt[j];
+        if (s[i] == s[j + 1]) j++;
+        nxt[i] = j;
+    }
+
+    for (int i = 1; i < s.size(); i++) {
+        bool x = s[i] - '0';
+        f[i - 1][x] = i;
+        p[i - 1] = f[i - 1][x ^ 1] = f[nxt[i - 1]][x ^ 1];
+    }
+
+    a[0] = 1;
+
+    for (int i = 1; i < s.size(); i++) {
+        a[i] = (static_cast<long long>(a[i - 1]) * 2 - a[p[i - 1]]) % mod;
+        b[i] = (static_cast<long long>(b[i - 1]) * 2 - b[p[i - 1]] - 2) % mod;
+    }
+
+    cout << (mod - (static_cast<long long>(b[s.size() - 1]) * binpow(a[s.size() - 1], mod - 2)) % mod) % mod << endl;
+
+    return 0;
+}