小易爱回文

• 逆序 原串，寻找 前缀，简单易懂

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        // 逆序原字符串
        String reverse = new StringBuffer(s).reverse().toString();
        String ans = "";
        int len = Integer.MAX_VALUE;

        if(judge(s)) {
            System.out.println(s);
        } else {
            for(int i=0;i<reverse.length();i++){
                // 寻找r对应s的前缀
                /**
                   i s(YCiC) r(CiCY)的前缀
                   0 YCiC    no
                   1 CiC     yes:CiC
                               CiC后追加(Y)
                               Y的index=r.length()-i
                 */
                if(reverse.startsWith(s.substring(i))){
                    String tmp = s+reverse.substring(reverse.length()-i);
                    if (tmp.length() < len) {
                        len = tmp.length();
                        ans = tmp;
                    }
                    // 为什么不直接输出呢？因为可能会有多个输出，我要输出最短的那个
                    // System.out.println(s+reverse.substring(reverse.length()-i));
                }
            }
             System.out.println(ans);
        }

    }

    public static boolean judge(String s) {
        int left = 0, right = s.length() - 1;
        while(left <= right) {
            if(s.charAt(left) == s.charAt(right)) {
                left++;
                right--;
            } else {
                return false;
            }
        }
        return true;
    }
}