枚举前缀哈希O1判断回文

int t;
    t =1;
    f[0]=1;
    for(int i=1;i<N;i++){
        f[i] = f[i-1]*ps;
    }
    while(t--){
        string s;
        cin >> s;
        if(s.size()==1){
            cout << s << endl;
            return 0;
        }
        ull p = 0,q = 0;
        int now = 0;
        for(int i=0;i<s.size();i++){
            p = p*ps + (s[i]-'a');
            q = 1ull*(s[i]-'a')*f[i]+q;
            if(p==q){
                now = max(i,now);
            }
        }
        for(int i=s.size()-1;i>now;i--){
            cout << s[i];
        }
        cout << s << endl;
    }
    return 0;

从右边枚举找到最短的回文串

import java.io.*;

public class Main {
    static BufferedReader bw = new BufferedReader(new InputStreamReader(System.in));
    static PrintWriter pw = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
    public static void main(String[] args) throws Exception{
        String str = bw.readLine();
       if(str.length() == 1) System.out.println(str);
       else {
           for (int i = str.length() - 1; i >= 0; i--) {
               if(aaa(str, i)){
                   pw.print(new StringBuffer(str.substring(i + 1)).reverse());
                   break;
               }
           }
           pw.println(str);
           pw.flush();
       }

    }
    public static boolean aaa(String str, int i){
        for (int j = 0; j <= i / 2; j++) {
            if(str.charAt(j) != str.charAt(i - j)) return false;
        }
        return true;
    }
}