Greatest english letter in upper and lower case

Hash Table String Enumeration

71.9% acceptance

Feb 25, 2026

522

Given a string of English letters s, return the greatest English letter which occurs as both a lowercase and uppercase letter in s. The returned letter should be in uppercase. If no such letter exists, return an empty string.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn greatest_letter(s: String) -> String {
    let mut lower = [false; 26];
    let mut upper = [false; 26];
    for c in s.chars() {
      if c.is_lowercase() {
        lower[(c as u8 - b'a') as usize] = true;
      } else {
        upper[(c as u8 - b'A') as usize] = true;
      }
    }
    for i in (0..26).rev() {
      if lower[i] && upper[i] {
        return ((b'A' + i as u8) as char).to_string();
      }
    }
    String::new()
  }
}