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Remove Duplicates Letters

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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.

#Solution 1
count = Counter(s) # initialize frequency hashmap
st = []
taken = set()
for char in s:
	if char not in taken:
		# check if there are any more instances of the evicting character
		while st and st[-1] > char and count[st[-1]] > 0:
	count[char] -= 1 # finish processing this character
return ''.join(st)

#Solution 2: Another solution written in slightly different way
class Solution:
    def removeDuplicateLetters(self, s: str) -> str:
        stack = []
        for idx, character in enumerate(s):
            if not stack:
            elif character in stack:
                while stack and (character < stack[-1]):
                    if stack[-1] in s[idx + 1:]:
                        _ = stack.pop()
        return ''.join(stack)

Another explanation of the same method

Posted by Jamie Meyer 3 months ago

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