55 lines
1.8 KiB
C++
55 lines
1.8 KiB
C++
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#include<bits/stdc++.h>
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using namespace std;
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#define mod(x, y) (((x) % (y) + (y)) % (y))
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using ll = int64_t;
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constexpr ll prime = 1e9 + 7;
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constexpr ll b1 = 825;
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constexpr ll b2 = 87825;
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class Solution {
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public:
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string longestPalindrome(string s) {
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int n = s.length();
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vector<ll> pa1(n + 1), pa2(n + 1);
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ll hash1 = 0, hash2 = 0;
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for (int i = 1; i <= n; ++i) {
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hash1 = (hash1 * b1 + s[i - 1]) % prime;
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hash2 = (hash2 * b2 + s[i - 1]) % prime;
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pa1[i] = hash1;
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pa2[i] = hash2;
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}
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vector<ll> sa1(n + 1), sa2(n + 1);
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hash1 = 0, hash2 = 0;
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for (int i = n - 1; i >= 0; --i) {
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hash1 = (hash1 * b1 + s[i]) % prime;
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hash2 = (hash2 * b2 + s[i]) % prime;
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sa1[i] = hash1;
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sa2[i] = hash2;
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}
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vector<ll> p1(n + 1), p2(n + 1);
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p1[0] = 1, p2[0] = 1;
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for (int i = 1; i < n; ++i) {
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p1[i] = (p1[i - 1] * b1) % prime;
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p2[i] = (p2[i - 1] * b2) % prime;
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}
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function<bool(int, int)> check_palindrome = [&] (int l, int r) {
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int p = l + r >> 1, q = l + r + 1 >> 1;
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return mod((pa1[p + 1] - pa1[l] * p1[p + 1 - l]), prime) == mod((sa1[q] - sa1[r + 1] * p1[r + 1 - q]), prime) &&
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mod((pa2[p + 1] - pa2[l] * p2[p + 1 - l]), prime) == mod((sa2[q] - sa2[r + 1] * p2[r + 1 - q]), prime);
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};
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int l = 0;
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pair<int, int> res;
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for (int i = 0; i < n; ++i) {
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l = max(0, l - 1);
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while (!check_palindrome(l, i)) ++l;
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res = max(res, {i - l + 1, l});
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}
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return s.substr(res.second, res.first);
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}
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};
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int main() {
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string s; cin >> s;
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cout << Solution().longestPalindrome(s);
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}
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