Largest palindromic number

Hash Table String Greedy Counting

37.0% acceptance

Feb 25, 2026

669

238

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.

Solution

Rust

Time O(n²)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn largest_palindromic(num: String) -> String {
    let mut freq = [0usize; 10];
    for c in num.bytes() { freq[(c - b'0') as usize] += 1; }
    let mut half: Vec<u8> = vec![];
    for d in (1..=9usize).rev() {
      for _ in 0..freq[d]/2 { half.push(b'0' + d as u8); }
    }
    if !half.is_empty() {
      for _ in 0..freq[0]/2 { half.push(b'0'); }
    }
    let middle = (0..=9usize).rev().find(|&d| freq[d] % 2 == 1).map(|d| b'0' + d as u8);
    if half.is_empty() {
      return String::from_utf8(vec![middle.unwrap_or(b'0')]).unwrap();
    }
    let mut result = half.clone();
    if let Some(m) = middle { result.push(m); }
    for &b in half.iter().rev() { result.push(b); }
    String::from_utf8(result).unwrap()
  }
}