LeetCode 316. Remove Duplicate Letters 棧頂

Given a string which contains only lowercase letters, 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: "bcabc"
Output: "abc"

Example 2:

Input: "cbacdcbc"
Output: "acdb"

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

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能想到用棧這題就不難：

from collections import defaultdict
class Solution:
    def removeDuplicateLetters(self, s: str) -> str:
        sta,exist = [],set()
        dic = {ch:idx for idx,ch in enumerate(s)}
        for idx,ch in enumerate(s):
            if (ch not in exist):
                while (sta and sta[-1]>ch and dic[sta[-1]]>idx):
                    exist.discard(sta[-1])
                    sta.pop()
                sta.append(ch)
                exist.add(ch)
        return ''.join(sta)

另外一種思路是從左到右找優先級最高的字符，如果這個字符是最後一次出現，那麼肯定應該放在這個位置。這個字符安置完，在原串中replace掉這個字符遞歸。

from collections import Counter

class Solution:
    def removeDuplicateLetters(self, s: str) -> str:

        c = Counter(s)
        pos = 0
        for i in range(len(s)):
            if s[i] < s[pos]: 
                pos = i
            c[s[i]] -=1
            if c[s[i]] == 0: 
                break
        return s[pos] + self.removeDuplicateLetters(s[pos:].replace(s[pos], "")) if s else ''