參考：https://www.cnblogs.com/grandyang/p/4475985.html#undefined

模板：

const int N = 1e5 + 10;
int p[N];
string manacher(string s) {
    string t = "$#";
    for (int i = 0; i < s.size(); ++i) {
        t += s[i];
        t += ‘#‘;
    }
    int mx = 0, id = 0, resl = 0, resc = 0;
    for (int i = 1
 
; i < t.size(); ++i) {
        p[i] = mx > i ? min(p[2*id-i], mx-i) : 1;
        while(t[i+p[i]] == t[i-p[i]]) ++p[i];
        if(mx < i+p[i]) mx = i+p[i], id = i;
        if(resl < p[i]) resl = p[i], resc = i;
    }
    return s.substr((resc - resl)/2, resl-1);
}

HDU 3068

代碼：

#pragma GCC optimize(2)
#pragma GCC optimize(3)
#pragma GCC optimize(4)
#include<bits/stdc++.h>
using namespace std;
#define y1 y11
#define fi first
#define se second
#define pi acos(-1.0)
#define LL long long
//#define mp make_pair
#define pb push_back
#define ls rt<<1, l, m
#define
 
 rs rt<<1|1, m+1, r
#define ULL unsigned LL
#define pll pair<LL, LL>
#define pli pair<LL, int>
#define pii pair<int, int>
#define piii pair<pii, int>
#define pdd pair<double, double>
#define mem(a, b) memset(a, b, sizeof(a))
#define debug(x) cerr << #x << " = " << x << "\n";
#define fio ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
//head

const int N = 3e5 + 10;
int p[N];
int manacher(string s) {
    string t = "$#";
    for (int i = 0; i < s.size(); ++i) {
        t += s[i];
        t += ‘#‘;
    }
    int mx = 0, id = 0, resl = 0, resc = 0;
    for (int i = 1; i < t.size(); ++i) {
        p[i] = mx > i ? min(p[2*id-i], mx-i) : 1;
        while(t[i+p[i]] == t[i-p[i]]) ++p[i];
        if(mx < i+p[i]) mx = i+p[i], id = i;
        if(resl < p[i]) resl = p[i], resc = i;
    }
    return resl-1;
}
int main() {
    string s;
    while(cin >> s) {
        cout << manacher(s) << endl;
    }
    return 0;
}

算法筆記--manacher算法