0316. Remove Duplicate Letters

https://leetcode.com/problems/remove-duplicate-letters

Description

Given a string s, remove duplicate letters so that every letter appears once and only once. You must make sure your result is the smallest in lexicographical order among all possible results.

Example 1:

**Input:** s = "bcabc"
**Output:** "abc"

Example 2:

**Input:** s = "cbacdcbc"
**Output:** "acdb"

Constraints:

• 1 <= s.length <= 104

• s consists of lowercase English letters.

Note: This question is the same as 1081: https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/

ac

class Solution {
    public String removeDuplicateLetters(String s) {
        // edge cases
        if (s.length() <= 1) return s;

        int n = s.length();
        int[] cnts = new int[26];
        for (char c : s.toCharArray()) {
            cnts[c - 'a']++;
        }
        boolean[] onStack = new boolean[26];
        Stack<Character> stack = new Stack<>();

        for (char c : s.toCharArray()) {
            cnts[c - 'a']--;
            if (onStack[c - 'a']) {
                continue; // skip
            }
            while (!stack.isEmpty() && c < stack.peek() && cnts[stack.peek() - 'a'] > 0) {
                onStack[stack.pop() - 'a'] = false;
            }
            stack.push(c);
            onStack[c - 'a'] = true;
        }

        StringBuilder sb = new StringBuilder();
        while (!stack.isEmpty()) {
            sb.append(stack.pop());
        }

        return sb.reverse().toString();
    }
}

/*
stack, walk string, if meet smaller char and old char has more later, pop old.
*/