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