Apply operations to make string empty

Array Hash Table Sorting Counting

57.4% acceptance

Feb 25, 2026

167

You are given a string s. Consider performing the following operation until s becomes empty: For every alphabet character from 'a' to 'z', remove the first occurrence of that character in s (if it exists). Return the value of the string s right before applying the last operation.

Solution

Rust

Time O(n)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn last_non_empty_string(s: String) -> String {
    let mut freq = [0u32; 26];
    for b in s.bytes() { freq[(b-b'a') as usize] += 1; }
    let max_f = *freq.iter().max().unwrap();
    // Characters that appear max_f times
    let _chars_in_last: Vec<u8> = (0..26u8).filter(|&c| freq[c as usize] == max_f).collect();
    // In the last operation, the last occurrence of each such char is the result
    let mut result: Vec<u8> = Vec::new();
    let mut seen = [false; 26];
    for b in s.bytes().rev() {
      let c = (b - b'a') as usize;
      if freq[c] == max_f && !seen[c] {
        seen[c] = true;
        result.push(b);
      }
    }
    result.reverse();
    String::from_utf8(result).unwrap()
  }
}