Largest Palindromic Number

class Solution {
public:
    string largestPalindromic(string num) {
        int cnt[10] = {0};
        for (char c : num) ++cnt[c - '0'];
        string half;
        for (int d = 9; d >= 0; --d) {
            while (cnt[d] >= 2) {
                if (half.empty() && d == 0) break; // avoid leading zeros
                half += '0' + d;
                cnt[d] -= 2;
            }
            if (half.empty() && d == 0) break;
        }
        int mid = -1;
        for (int d = 9; d >= 0; --d) if (cnt[d] > 0) { mid = d; break; }
        string ans = half;
        if (mid >= 0) ans += '0' + mid;
        ans += string(half.rbegin(), half.rend());
        return ans.empty() ? "0" : ans;
    }
};

func largestPalindromic(num string) string {
    cnt := [10]int{}
    for _, c := range num {
        cnt[c-'0']++
    }
    half := []byte{}
    for d := 9; d >= 0; d-- {
        for cnt[d] >= 2 {
            if len(half) == 0 && d == 0 {
                break
            }
            half = append(half, byte('0'+d))
            cnt[d] -= 2
        }
        if len(half) == 0 && d == 0 {
            break
        }
    }
    mid := -1
    for d := 9; d >= 0; d-- {
        if cnt[d] > 0 {
            mid = d
            break
        }
    }
    ans := append([]byte{}, half...)
    if mid >= 0 {
        ans = append(ans, byte('0'+mid))
    }
    for i := len(half) - 1; i >= 0; i-- {
        ans = append(ans, half[i])
    }
    if len(ans) == 0 {
        return "0"
    }
    return string(ans)
}

class Solution:
    def largestPalindromic(self, num: str) -> str:
        cnt = [0] * 10
        for c in num:
            cnt[int(c)] += 1
        half = []
        for d in range(9, -1, -1):
            while cnt[d] >= 2:
                if not half and d == 0:
                    break
                half.append(str(d))
                cnt[d] -= 2
            if not half and d == 0:
                break
        mid = -1
        for d in range(9, -1, -1):
            if cnt[d] > 0:
                mid = d
                break
        ans = ''.join(half)
        if mid >= 0:
            ans += str(mid)
        ans += ''.join(reversed(half))
        return ans if ans else "0"

function largestPalindromic(num) {
    const cnt = new Array(10).fill(0);
    for (const c of num) cnt[c.charCodeAt(0) - 48]++;
    let half = '';
    for (let d = 9; d >= 0; d--) {
        while (cnt[d] >= 2) {
            if (half.length === 0 && d === 0) break;
            half += String(d);
            cnt[d] -= 2;
        }
        if (half.length === 0 && d === 0) break;
    }
    let mid = -1;
    for (let d = 9; d >= 0; d--) {
        if (cnt[d] > 0) { mid = d; break; }
    }
    let ans = half;
    if (mid >= 0) ans += String(mid);
    ans += half.split('').reverse().join('');
    return ans === '' ? '0' : ans;
}

class Solution {
    public String largestPalindromic(String num) {
        int[] cnt = new int[10];
        for (int i = 0; i < num.length(); i++) cnt[num.charAt(i) - '0']++;
        StringBuilder half = new StringBuilder();
        for (int d = 9; d >= 0; d--) {
            while (cnt[d] >= 2) {
                if (half.length() == 0 && d == 0) break;
                half.append((char) ('0' + d));
                cnt[d] -= 2;
            }
            if (half.length() == 0 && d == 0) break;
        }
        int mid = -1;
        for (int d = 9; d >= 0; d--) {
            if (cnt[d] > 0) { mid = d; break; }
        }
        StringBuilder ans = new StringBuilder(half);
        if (mid >= 0) ans.append((char) ('0' + mid));
        ans.append(half.reverse());
        return ans.length() == 0 ? "0" : ans.toString();
    }
}

function largestPalindromic(num: string): string {
    const cnt: number[] = new Array(10).fill(0);
    for (const c of num) cnt[c.charCodeAt(0) - 48]++;
    let half = '';
    for (let d = 9; d >= 0; d--) {
        while (cnt[d] >= 2) {
            if (half.length === 0 && d === 0) break;
            half += String(d);
            cnt[d] -= 2;
        }
        if (half.length === 0 && d === 0) break;
    }
    let mid = -1;
    for (let d = 9; d >= 0; d--) {
        if (cnt[d] > 0) { mid = d; break; }
    }
    let ans = half;
    if (mid >= 0) ans += String(mid);
    ans += half.split('').reverse().join('');
    return ans === '' ? '0' : ans;
}