模板——manacher（马拉车）

#include<bits/stdc++.h>
using namespace std;
string rem,s;
int pos=0,maxright=0,maxlen=0,r[5000005]={0};
int main(){
	cin>>rem;
	for (int i=0; i<rem.length(); i++){
		s+='#';
		s+=rem[i];
	}
	s+='#';
	for (int i=0; i<s.length(); i++){
		if (i<maxright)
			r[i]=min(r[2*pos-i],maxright-i);
		else r[i]=1;
		while (i-r[i]>=0 && i+r[i]<s.length() && s[i-r[i]]==s[i+r[i]])
			r[i]++;
		if (r[i]+i-1>maxright){
			maxright=r[i]+i-1;
			pos=i;
		}
			maxlen=max(maxlen,r[i]);
	}
	cout<<maxlen-1;
	return 0;
}