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Java-based LeetCode algorithm problem solutions, regularly updated

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package g1001_1100.s1081_smallest_subsequence_of_distinct_characters;

// #Medium #String #Greedy #Stack #Monotonic_Stack
// #2022_02_24_Time_2_ms_(97.16%)_Space_40.2_MB_(41.62%)

import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Deque;

/**
 * 1081 - Smallest Subsequence of Distinct Characters\.
 *
 * Medium
 *
 * Given a string `s`, return _the lexicographically smallest subsequence of_ `s` _that contains all the distinct characters of_ `s` _exactly once_.
 *
 * **Example 1:**
 *
 * **Input:** s = "bcabc"
 *
 * **Output:** "abc"
 *
 * **Example 2:**
 *
 * **Input:** s = "cbacdcbc"
 *
 * **Output:** "acdb"
 *
 * **Constraints:**
 *
 * *   `1 <= s.length <= 1000`
 * *   `s` consists of lowercase English letters.
 *
 * **Note:** This question is the same as 316: [https://leetcode.com/problems/remove-duplicate-letters/](https://leetcode.com/problems/remove-duplicate-letters/)
**/
public class Solution {
    public String smallestSubsequence(String s) {
        int n = s.length();
        Deque stk = new ArrayDeque<>();
        int[] freq = new int[26];
        boolean[] exist = new boolean[26];
        Arrays.fill(exist, false);
        for (char ch : s.toCharArray()) {
            freq[ch - 'a']++;
        }
        for (int i = 0; i < n; i++) {
            char ch = s.charAt(i);
            freq[ch - 'a']--;
            if (exist[ch - 'a']) {
                continue;
            }
            while (!stk.isEmpty() && stk.peek() > ch && freq[stk.peek() - 'a'] > 0) {
                char rem = stk.pop();
                exist[rem - 'a'] = false;
            }
            stk.push(ch);
            exist[ch - 'a'] = true;
        }
        char[] ans = new char[stk.size()];
        int index = 0;
        while (!stk.isEmpty()) {
            ans[index] = stk.pop();
            index++;
        }
        return new StringBuilder(new String(ans)).reverse().toString();
    }
}