Largest number after digit swaps by parity

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Feb 25, 2026

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You are given a positive integer num. You may swap any two digits of num that have the same parity (i.e. both odd digits or both even digits). Return the largest possible value of num after any number of swaps.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn largest_integer(num: i32) -> i32 {
    let mut digits: Vec<u32> = num.to_string().chars().map(|c| c.to_digit(10).unwrap()).collect();
    let mut odds: Vec<u32> = digits.iter().filter(|&&d| d % 2 == 1).cloned().collect();
    let mut evens: Vec<u32> = digits.iter().filter(|&&d| d % 2 == 0).cloned().collect();
    odds.sort_unstable_by(|a, b| b.cmp(a));
    evens.sort_unstable_by(|a, b| b.cmp(a));
    let mut oi = 0;
    let mut ei = 0;
    for d in &mut digits {
      if *d % 2 == 1 {
        *d = odds[oi];
        oi += 1;
      } else {
        *d = evens[ei];
        ei += 1;
      }
    }
    digits.iter().fold(0i32, |acc, &d| acc * 10 + d as i32)
  }
}