2023-09-26 10:43:09 +0000 UTC

class Solution:
    def removeDuplicateLetters(self, s: str) -> str:
        stack = []
        seen = set() 
        last_occ = {char: i for i, char in enumerate(s)}
        
        for i, char in enumerate(s):
            if char in seen:
                continue
                
            while stack and char < stack[-1] and i < last_occ[stack[-1]]:
                seen.discard(stack.pop())
            seen.add(char)
            stack.append(char)
        
        return ''.join(stack)