题解 | #密码截取#

下面这种写法结果虽然正确，但是字符串太长时，计算太耗时，不符合题目要求。

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNext()) {
            String str = sc.nextLine();
            int max = 0;
            int length = str.length();
            for (int i = 0; i < length; i++) {
                for (int j = length; j > i; j--) {
                    if (isSymmetry(str, i, j)) {
                        max = Math.max(max, j - i);
                    }
                }
            }
            System.out.println(max);

        }
        sc.close();
    }

    private static boolean isSymmetry(String str, int i, int j) {
        String sub = str.substring(i, j);
        StringBuilder sb = new StringBuilder(sub);
        return sb.reverse().toString().equals(sub);
    }
}

最优写法：

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        System.out.println(solution(s));
    }

    private static int solution(String s) {
        int res = 0;
        for(int i = 0; i < s.length(); i++) {
            // ABA型
            int len1 = longest(s, i, i);
            // ABBA型
            int len2 = longest(s, i, i + 1);
            res = Math.max(res, len1 > len2 ? len1 : len2);
        }
        return res;
    }

    private static int longest(String s, int l, int r) {
        while(l >= 0 && r < s.length() && s.charAt(l) == s.charAt(r)) {
            l--;
            r++;
        }
        return r - l - 1;
    }
}