#1728. 【2022.11.02 联考】字符串还原（friends）

https://sjzezoj.com/submission/63180

S2OJ/1728/1728.cpp

@@ -0,0 +1,83 @@
+#include <iostream>
+#include <string>
+#include <unordered_set>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 2e6 + 5;
+
+class Hash {
+  private:
+    unsigned long long hash[N], p[N];
+
+  public:
+    Hash(std::string s = "", int base = 131) {
+        p[0] = 1;
+
+        for (int i = 1; i <= s.size(); i++) {
+            hash[i] = hash[i - 1] * base + s[i - 1];
+            p[i] = p[i - 1] * base;
+        }
+    }
+
+    unsigned long long get(int l, int r) {
+        return hash[r] - hash[l - 1] * p[r - l + 1];
+    }
+
+    const unsigned long long &operator[](int i) const {
+        return p[i];
+    }
+};
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    int n;
+    std::string u;
+
+    cin >> n >> u;
+
+    if (n % 2 == 0) {
+        cout << "NOT POSSIBLE" << endl;
+
+        exit(0);
+    }
+
+    Hash hash(u, 131);
+    int mid = n >> 1;
+    unsigned long long lst = 0;
+    std::string ans;
+
+    for (int i = 1; i <= n; i++) {
+        if (i - 1 <= mid) {
+            if (hash.get(1, i - 1) * hash[mid - i + 1] + hash.get(i + 1, mid + 1) == hash.get(mid + 2, n)) {
+                if (lst && lst != hash.get(mid + 2, n)) {
+                    cout << "NOT UNIQUE" << endl;
+
+                    exit(0);
+                } else if (!lst) {
+                    ans = u.substr(mid + 1, mid);
+                    lst = hash.get(mid + 2, n);
+                }
+            }
+        } else {
+            if (hash.get(1, mid) == hash.get(mid + 1, i - 1) * hash[n - i] + hash.get(i + 1, n)) {
+                if (lst && lst != hash.get(1, mid)) {
+                    cout << "NOT UNIQUE" << endl;
+
+                    exit(0);
+                } else if (!lst) {
+                    ans = u.substr(0, mid);
+                    lst = hash.get(1, mid);
+                }
+            }
+        }
+    }
+
+    cout << (ans.empty() ? "NOT POSSIBLE" : ans) << endl;
+
+    return 0;
+}