蒟蒻manacher92pts求助

WA了第二个点qwq，调了好久了

代码：

#include<bits/stdc++.h>
#define N 1000010
using namespace std;
int n,f[N],ans;
char s[N];
void manacher(int x){
    int mid=0,maxn=0;
    for(int i=1;i<=x;i+=2){
        f[i]=(maxn>i)?min(f[2*mid-i],maxn-i):1;
        if(maxn>i&&i-f[i]<mid){
            ans=max(ans,2*(i-mid));
        }
        while(s[i+f[i]]==s[i-f[i]]){
            f[i]++;
        }
        if(f[i]+i>maxn){
            mid=i,maxn=f[i]+i;
        }
    }
}
int read(){
    int x=0;
    char ch=getchar();
    while(ch<'0'||ch>'9'){
        ch=getchar();
    }
    while(ch>='0'&&ch<='9'){
        x=(x<<3)+(x<<1)+ch-'0';
        ch=getchar();
    }
    return x;
}
int main(){
    n=read();
    scanf("%s",s+1);
    s[1]='$';
    for(int i=n;i;i--){
        s[i*2+1]='$',s[i*2]=s[i];
    }
    manacher(2*n);
    printf("%d",ans);
    return 0;
}