#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int maxn=11000000;
char s[maxn+10],a[(maxn<<1)+10]; int l,n,maxv,pos,rl[(maxn<<1)+10],ans;
int main(){
    scanf("%s",s); l=strlen(s);
    for(register int i=0;i<l;++i){a[++n]=s[i]; ++n;}
    maxv
 
=pos=0; rl[0]=1;
    for(register int i=1;i<=n;++i){
        if(i<=maxv) rl[i]=min(rl[(pos<<1)-i],maxv-i);
        else rl[i]=1;
        for(;i+rl[i]<=n&&i-rl[i]>=0&&a[i-rl[i]]==a[i+rl[i]];++rl[i]);
        if(i+rl[i]-1>maxv){
            maxv=i+rl[i]-1; pos=i;
        }
        ans
 
=max(ans,rl[i]-1);
    }
    printf("%d",ans); return 0;
}

Manacher算法(最長回文子串)