拉拉队排练(Cheerleading Rehearsal) 题解

思路分析

按题目要求使用对应数据结构与算法模拟/递推求解，注意数据范围边界与时间复杂度。

参考代码

#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
using namespace std;

const long long MOD = 19930726;

long long qpow(long long a, long long b) {
    long long res = 1;
    a %= MOD;
    while (b > 0) {
        if (b & 1) res = res * a % MOD;
        a = a * a % MOD;
        b >>= 1;
    }
    return res;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n;
    long long K;
    if (!(cin >> n >> K)) return 0;
    string s;
    cin >> s;
    vector<int> d1(n, 0);
    int l = 0, r = -1;
    vector<long long> cnt(n + 2, 0);
    for (int i = 0; i < n; ++i) {
        int k = (i > r) ? 1 : min(d1[l + r - i], r - i + 1);
        while (0 <= i - k && i + k < n && s[i - k] == s[i + k]) ++k;
        d1[i] = k--;
        if (i + k > r) {
            l = i - k;
            r = i + k;
        }
        int max_len = 2 * d1[i] - 1;
        cnt[max_len]++;
    }
    long long ans = 1;
    long long cur = 0;
    for (int len = (n % 2 == 1 ? n : n - 1); len >= 1; len -= 2) {
        cur += cnt[len];
        if (cur == 0) continue;
        long long take = min(K, cur);
        ans = ans * qpow(len, take) % MOD;
        K -= take;
        if (K == 0) break;
    }
    if (K > 0) cout << -1 << "\n";
    else cout << ans << "\n";
    return 0;
}