Learning On the Way

316. Remove Duplicate 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.

def removeDuplicateLetters(self, s: str) -> str: 
    ct = collections.Counter(s)              
    stack = [] 
    seen = set() 
     
    for i, c in enumerate(s): 
        if c in seen: 
            ct[c] -= 1 
            continue 
        while stack and s[stack[-1]] >= c and ct[s[stack[-1]]] > 1: 
            idx = stack.pop() 
            seen.remove(s[idx]) 
            ct[s[idx]] -= 1 
        stack.append(i) 
        seen.add(c) 
         
    return ''.join([s[i] for i in stack])