Remove Duplicates Letters
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:
taken.remove(st.pop())
st.append(char)
taken.add(char)
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:
stack.append(character)
elif character in stack:
continue
else:
while stack and (character < stack[-1]):
if stack[-1] in s[idx + 1:]:
_ = stack.pop()
else:
break
stack.append(character)
return ''.join(stack)
Another explanation of the same method
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