Remove duplicate letters

class Solution:
    def removeDuplicateLetters(self, s: str) -> str:
        stk = []
        count = [0] * 256
        for i in range(len(s)):
            count[ord(s[i])] += 1
        in_stack = [False] * 256
        for c in s:
            count[ord(c)] -= 1
            if in_stack[ord(c)]:
                continue
            while stk and stk[-1] > c:
                if count[ord(stk[-1])] == 0:
                    break
                in_stack[ord(stk.pop())] = False
            stk.append(c)
            in_stack[ord(c)] = True

        return "".join(stk)

Remove Duplicate Letters

Difficulty: Medium

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/