最大回文数字

/*
 * Approach using Java Streams:
 * 1. Count the frequency of each digit using groupingBy and counting.
 * 2. Build the half palindrome by flatMapping digits.
 * 3. Determine if there is a center digit with odd frequency.
 * 4. Combine the half, center (if any), and reversed half to form the full palindrome.
 */

import java.util.Map;
import java.util.stream.Collectors;
import java.util.function.Function;
import java.util.Comparator;
import java.util.LinkedHashMap;
import java.util.stream.IntStream;

public class MaxPalindromeStream {
    public static String largestPalindrome(String num) {
        // Count frequency of each digit
        Map<Character, Long> count = num.chars()
                .mapToObj(c -> (char) c)
                .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

        // Construct the half palindrome and find the center if any
        StringBuilder half = new StringBuilder();
        Character center = null;

        count = count.entrySet().stream()
                .sorted(Map.Entry.<Character, Long>comparingByKey(Comparator.reverseOrder()))
                .collect(Collectors.toMap(
                        Map.Entry::getKey, Map.Entry::getValue,
                        (e1, e2) -> e1, LinkedHashMap::new));

        for (Map.Entry<Character, Long> entry : count.entrySet()) {
            char digit = entry.getKey();
            long cnt = entry.getValue();
            if (cnt % 2 == 1 && (center == null || digit > center)) {
                center = digit;
            }
            half.append(String.valueOf(digit).repeat((int) (cnt / 2)));
        }

        // Avoid leading zeroes
        if (half.length() == 0 && center != null && center == '0') {
            return "0";
        }

        // Construct the largest palindrome
        String halfStr = half.toString();
        String palindrome = halfStr;
        if (center != null) {
            palindrome += center;
        }
        palindrome += new StringBuilder(halfStr).reverse().toString();
        return palindrome;
    }

    public static void main(String[] args) {
        System.out.println(largestPalindrome("444947137")); // Output: 7449447
        System.out.println(largestPalindrome("00009")); // Output: 9
    }
}