"蔚来杯"2022牛客暑期多校训练营9 G Magic Spells【马拉车+哈希】

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hash求子串可以O(1)，

$click$ $for$ $codes$

# include "bits/stdc++.h"

using namespace std;

constexpr int N = 3e5 + 3;

char str[N], str_new[N << 1];

int main() {

    ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);

    int n;

    cin >> n;

    unordered_map<unsigned long long, int> mp;

    unordered_map<unsigned long long, int> mp2;

    unordered_map<unsigned long long, bool> vis[6];

    unordered_map<unsigned long long, bool> vis2[6];

    int ans = 0;

    str_new[0] = '!';

    str_new[1] = '@';

    for(int i = 1; i <= n; ++i) {

        cin >> str + 1;

        int len = strlen(str + 1);

        int len_new = 1;

        for(int j = 1; j <= len; ++j) {

            str_new[++len_new] = str[j];

            str_new[++len_new] = '@';

        }

        str_new[++len_new] = '#';

        int r = 0, center;

        vector<int> p(len_new + 1 << 1);

        vector<int> pos;

        for(int j = 1; j <= len_new; ++j) {

            p[j] = (j < r) ? min(p[(center << 1) - j], r - j) : 1;

            while(str_new[j - p[j]] == str_new[j + p[j]]) ++p[j];

            if(r < j + p[j]) {

                r = j + p[j];

                center = j;

            }

            if(p[j] > 1) pos.push_back(j);

        }

        constexpr unsigned long long base = 122777, mod = 91815541, mod2 = 19260817; // hash value, which is indescibable

        vector<unsigned long long> h(len + 1), b(len + 1), h2(len + 1), b2(len + 1);

        b[0] = 1, b2[0] = 1;

        for(int i = 1; i <= len; ++i) {

        	h[i] = (h[i - 1] * base % mod + (unsigned long long) str[i]) % mod;

        	h2[i] = (h2[i - 1] * base % mod2 + (unsigned long long) str[i]) % mod2;

        	b[i] = b[i - 1] * base % mod;

        	b2[i] = b2[i - 1] * base % mod2;

		}

		auto hash = [&](int l, int r) -> unsigned long long {

			return (h[r] - h[l - 1] * (unsigned long long)b[r - l + 1] % mod + mod) % mod;

		};

		auto hash2 = [&](int l, int r) -> unsigned long long { // double hash reduces chance and thus improves accuracy

			return (h2[r] - h2[l - 1] * (unsigned long long)b2[r - l + 1] % mod2 + mod2) % mod2;

		};

        for(auto j : pos) {

            int len = p[j];

            while(len) {

            	if((j - len + 2 >> 1) > (j + len - 2 >> 1)) break; // from several manual simulations, we'll find that the palindrome string centered on i in the new string is (i - p[i】 + 2) / 2 ---> (i + p[i] - 2) / 2 in the corresponding position in the original string

                unsigned long long sub_str_1 = hash( (j - len + 2 >> 1), (j + len - 2 >> 1) );

                unsigned long long sub_str_2 = hash2( (j - len + 2 >> 1), (j + len - 2 >> 1) );

                if(vis[i][sub_str_1] == true && vis2[i][sub_str_2] == true) break;

                vis[i][sub_str_1] = vis2[i][sub_str_2] = true;

                ++mp[sub_str_1], ++mp2[sub_str_2];

                if(mp[sub_str_1] >= n && mp2[sub_str_2] >= n) ++ ans;

                len -= 2; // manachar only directly finds the longest palindrome string, but the string may contain other substrings

                if(len < 0) break;

            }

        }

    }

    cout << ans;

    return 0;

}

/*

9999

abcba

abcccba

*/

巴特西

"蔚来杯"2022牛客暑期多校训练营9 G Magic Spells【马拉车+哈希】

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