最长双回文串(Longest Double Palindrome) 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;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    string s;
    if (!(cin >> s)) return 0;
    string t = "^#";
    for (char c : s) {
        t += c;
        t += '#';
    }
    t += '$';
    int m = (int)t.size();
    vector<int> p(m, 0), L(m, 0), R(m, 0);
    int c = 0, r = 0;
    for (int i = 1; i < m - 1; ++i) {
        int i_mirror = 2 * c - i;
        if (r > i) p[i] = min(r - i, p[i_mirror]);
        while (t[i + 1 + p[i]] == t[i - 1 - p[i]]) ++p[i];
        if (i + p[i] > r) {
            c = i;
            r = i + p[i];
        }
        L[i + p[i]] = max(L[i + p[i]], p[i]);
        R[i - p[i]] = max(R[i - p[i]], p[i]);
    }
    for (int i = m - 3; i >= 1; i -= 2) L[i] = max(L[i], L[i + 2] - 2);
    for (int i = 3; i < m - 1; i += 2) R[i] = max(R[i], R[i - 2] - 2);
    int ans = 0;
    for (int i = 3; i < m - 2; i += 2) {
        if (L[i] > 0 && R[i] > 0) ans = max(ans, L[i] + R[i]);
    }
    cout << ans << "\n";
    return 0;
}