README.md

2384. Largest Palindromic Number

You are given a string num consisting of digits only.

Return the largest palindromic integer (in the form of a string) that can be formed using digits taken from num. It should not contain leading zeroes.

Notes:

• You do not need to use all the digits of num, but you must use at least one digit.
• The digits can be reordered.

Example 1:

Input: num = "444947137"
Output: "7449447"
Explanation:
Use the digits "4449477" from "444947137" to form the palindromic integer "7449447".
It can be shown that "7449447" is the largest palindromic integer that can be formed.

Example 2:

Input: num = "00009"
Output: "9"
Explanation:
It can be shown that "9" is the largest palindromic integer that can be formed.
Note that the integer returned should not contain leading zeroes.

Constraints:

• 1 <= num.length <= 105
• num consists of digits.

Solutions (Rust)

1. Solution

impl Solution {
    pub fn largest_palindromic(num: String) -> String {
        let mut count = [0; 10];
        let mut digits = vec![];

        for digit in num.bytes() {
            count[(digit - b'0') as usize] += 1;
        }

        for i in (0..=9).rev() {
            while count[i] > 1 {
                count[i] -= 2;
                digits.push(i as u8 + b'0');
            }
        }

        if *digits.get(0).unwrap_or(&b'1') == b'0' {
            digits.clear();
        }

        let mut digits_rev = digits.clone();
        digits_rev.reverse();

        if let Some(i) = (0..=9).rev().find(|&i| count[i] > 0) {
            digits.push(i as u8 + b'0');
        }
        digits.append(&mut digits_rev);
        if digits.is_empty() {
            digits.push(b'0');
        }

        String::from_utf8(digits).unwrap()
    }
}