51nod 1089最长回文子串V2 (manacher)

经典题

manacher是一种很神奇的算法，

算是动态规划的一种，不过利用的信息非常有效

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
const int maxn = 1e5 + ;
char S2[maxn*], str[maxn*];
int R[maxn*];
void Manacher(char* S){
    int L = strlen(S);
    for(int i = , j = ; i < *L; i += , j += ) S2[i] = '#', S2[j] = S[j/];
    S2[*L] = '#';
    int mx = -, id = -;
    for(int i = ; i < *L; i++){
        R[i] = mx-i >=  ? min(R[*id-i], mx-i+) : ;
        while(i+R[i] <= *L && S2[i+R[i]] == S2[i-R[i]]) R[i]++;
        if(i+R[i]- > mx) mx = i+R[i]-, id = i;
    }
}

int main()
{
    cin>>str;
    Manacher(str);
    int L = strlen(str), ans = ;
    //for(int i = 0; i < 2*L; i++) cout<<R[i]<<" "; cout<<endl;
    for(int i = ; i < *L; i += ) ans = max(ans, *(R[i]/-) + );
    for(int i = ; i < *L; i += ) ans = max(ans, R[i]-);
    cout<<ans<<endl;
    return ;
}