動機

保持原本的順序，維持字典序(小於等於) Monotonic Stack

Problem

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/

Sol

Monotonic Stack要用但是有一個限制，只能移除多的字元，所以要移除時要看後面還有沒有其他機會

class Solution:
    def removeDuplicateLetters(self, s: str) -> str:
        cnts = Counter(s) # 還有幾次可以選
        stk = []
        for c in s:
            cnts[c] -= 1
            if c not in stk:
                while stk and stk[-1] >= c and cnts[stk[-1]] > 0:
                    stk.pop()
                stk += c,
        return ''.join(stk)