题解 | #密码截取# 分类讨论,使用一维dp数组

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String str = in.nextLine();
        char[] arr = str.toCharArray();
        int n = arr.length;
        int[] dp = new int[n];//dp[i]表示arr[i]结尾的最大有效密码串长度
        //初始化
        for (int i = 0; i < n; i++) {
            dp[i] = 1;
        }
        int max = 0;
		//一大堆分类讨论,比较繁琐,因为一开始没想出原来用二维dp数组
        for (int i = 1; i < n; i++) {

            if (dp[i - 1] == 1) {
                //AA
                if (arr[i] == arr[i - 1]) {
                    dp[i] = 2;
                }
                //ABA
                if (i - 2 >= 0 && arr[i] == arr[i - 2]) {
                    dp[i] = 3;
                }
            } else {
                //AAA
                if (i - 2 >= 0 && arr[i] == arr[i - 2]) {
                    dp[i] = 3;
                }
                int left = i - dp[i - 1] - 1;
                //AAAA
                if (dp[i - 1] == 3 && arr[i - 1] == arr[i - 2] && arr[i] == arr[i - 1]) {
                    dp[i] = 4;
                }
                //通用情况, A[...]A
                if (left >= 0 && arr[left] == arr[i]) {
                    dp[i] = dp[i - 1] + 2;
                }
            }
            max = Math.max(max, dp[i]);
        }
        System.out.println(max);
    }
}