洛谷 P3805 manacher算法

链接：https://www.luogu.org/problemnew/show/P3805

思路：模板，直接上马拉车

代码：

//#include<bits/stdc++.h>
#include<iostream>
#include<vector>
#include<stack>
#include<string>
#include<cstdio>
#include<algorithm>
#include<queue>
#include<map>
#include<set>
#include<cmath>
#define inf 0x3f3f3f3f
using namespace std;
typedef long long ll;
const ll M = ll(1e7) * 3 + 5;
string s;
string s2;
ll len[M];
string init(string s)
{
    s2.clear();
    s2 += '@';
    for (ll i = 0; i < s.size(); i++)
    {
        s2 += '#';
        s2 += s[i];
    }
    //s2 += s[s.size() - 1];
    s2 += '#';
//    s2 += '$';
//    s2 += '';
    return s2;
}
ll Manacher(string s)
{
    ll p0 = 0, mx = 0, ans = 0;
    for (ll i = 1; i < s.size(); i++)
    {
        if (i < mx)
            len[i] = min(mx - i, len[2 * p0 - i]);
        else
            len[i] = 1;
        while (s[i - len[i]] == s[i + len[i]])
            len[i]++;
        if (i + len[i] > mx)
        {
            mx = len[i] + i;
            p0 = i;
        }
        ans = max(ans, len[i]);
    }
    return ans - 1;
}
int main()
{
    while (cin >> s)
    {
        s = init(s);
        //cout << s << endl;
        cout << Manacher(s) << endl;
    }
    return 0;
}

备注：在初始化原串的时候要注意还需要再加一个不同的字符，我这里用的是@，防止马拉车的时候上溢，详情可见这篇博客