316. Remove Duplicate Letters

Problem Link

Description

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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/

Solution

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Python3

class Solution:
    def removeDuplicateLetters(self, s: str) -> str:
        count = Counter(s)
        stack = []
        put = set()
 
        for i, x in enumerate(s):
            count[x] -= 1
            if x in put: continue
 
            while stack and x < stack[-1] and count[stack[-1]] > 0:
                put.remove(stack.pop())
            
            stack.append(x)
            put.add(x)
 
        return "".join(stack)