leetcode/1081: monotonic stack

[tony84727]

No description provided.

[gemini-code-assist]

Code Review

This pull request adds a Rust solution for LeetCode problem 1081, "Smallest Subsequence of Distinct Characters", including tests and a README. The reviewer suggested optimizing the solution by working directly with bytes (u8) instead of char to avoid UTF-8 decoding overhead and simplify the logic.

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[gemini-code-assist]

Since the input string s is guaranteed to consist of lowercase English letters (ASCII), we can optimize the implementation by working directly with bytes (u8) instead of char. This avoids the overhead of UTF-8 decoding in char_indices() and simplifies the code by removing the need for Option wrapping in last_index and the helper index function.

impl Solution {
    pub fn smallest_subsequence(s: String) -> String {
        let bytes = s.as_bytes();
        let mut last_index = [0; 26];
        for (i, &b) in bytes.iter().enumerate() {
            last_index[(b - b'a') as usize] = i;
        }
        let mut used = [false; 26];
        let mut answer: Vec<u8> = vec![];
        for (i, &b) in bytes.iter().enumerate() {
            let bi = (b - b'a') as usize;
            if used[bi] {
                continue;
            }
            while let Some(&top) = answer.last() {
                let ti = (top - b'a') as usize;
                if top > b && last_index[ti] > i {
                    answer.pop();
                    used[ti] = false;
                } else {
                    break;
                }
            }
            used[bi] = true;
            answer.push(b);
        }
        String::from_utf8(answer).unwrap()
    }