求问

#include <bits/stdc++.h>
using namespace std;
int n,Next[800005];
string s;
signed main() {
    cin>>n;
    for (int i=n+1;i<=2*n;i++) cin>>s[i];
    s[n]='$';
    for (int i=0;i<n;i++) s[i]=s[2*n-i];
    for (int i=1;i<=2*n;i++) {
        int j=Next[i];
        while (j&&s[i]!=s[j]) j=Next[j];
        if (s[i]==s[j]) Next[i+1]=j+1;
        else Next[i+1]=0;
    }
    cout<<n-Next[2*n+1];
    return 0;
}