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下面仅为自写代码

一、朴素做法

import java.util.*;

public class Main {
    static int n, ans = 0;
    static String str;

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        str = sc.nextLine();
        n = str.length();
        for (int i = 0; i < n; i++) {
            int l = i, r = i;
            up(i, i);
        }
        for (int i = 1; i < n; i++) {
            if (str.charAt(i) == str.charAt(i - 1)) {
                up(i - 1, i);
            }
        }

        System.out.println(ans);
    }

    static void up(int l, int r) {
        while (check(l, r)) {
                l--;
                r++;
            }
        if (l < 0 || r >= n) {
            l ++;
            r --;
        }
        if (l >= 0 && r < n && str.charAt(l) != str.charAt(r)) {
            l ++;
            r --;
        }
        ans = Math.max(ans, r - l + 1);
    }

    static boolean check(int l, int r) {
        return l >= 0 && r < n && str.charAt(l) == str.charAt(r);
    }
}

二、字符串哈希与二分

import java.util.*;

public class Main {
    static final int N = 1010, p = 131, mod = 998244353;
    static long[] pre_h = new long[N], suf_h = new long[N], exp = new long[N];
    static int n, ans = 1;
    static String str;

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        str = " " + sc.nextLine();
        n = str.length() - 1;
        exp[0] = 1;
        for (int i = 1; i <= n; i++) {
            exp[i] = exp[i - 1] * p % mod;
            pre_h[i] = (pre_h[i - 1] * p + str.charAt(i)) % mod;
        }
        for (int i = n; i > 0; i--) {
            suf_h[i] = (suf_h[i + 1] * p + str.charAt(i)) % mod;
        }

        for (int i = 1; i <= n - 1; i++) {
            int l = 0, r = Math.min(i - 1, n - i);//半径
            while (l < r) {
                int mid = l + r + 1>> 1;
                if (check(i - mid, i - 1, i + 1, i + mid)) l = mid;
                else r = mid - 1;
            }
            ans = Math.max(ans, 2 * l + 1);
        }

        for (int i = 2; i <= n - 2; i++) {
            if (str.charAt(i) == str.charAt(i + 1)) {
                int l = 0, r = Math.min(i - 1, n - i - 1);
                while (l < r) {
                    int mid = l + r + 1 >> 1;
                    if (check(i - mid, i - 1, i + 2, i + mid + 1)) l = mid;
                    else r = mid - 1;
                }
                ans = Math.max(ans, 2 * l + 2);
            }
        }

        System.out.println(ans);
    }

    static boolean check(int l1, int r1, int l2, int r2) {
        long h1 = getMod(pre_h[r1] - pre_h[l1 - 1] * exp[r1 - l1 + 1]);
        long h2 = getMod(suf_h[l2] - suf_h[r2 + 1] * exp[r2 - l2 + 1]);
        return h1 == h2;
    }

    static long getMod(long u) {
        return (u % mod + mod) % mod;
    }
}