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.
Input: "bcabc" Output: "abc"
Input: "cbacdcbc" Output: "acdb"
class Solution: def removeDuplicateLetters(s): from collections import defaultdict last_index = defaultdict(int) for i, ch in enumerate(s): last_index[ch] = i ret = '' for i, ch in enumerate(s): if ch not in ret: # If curr can be put before the last char while len(ret) and ch < ret[-1] \ and i < last_index[ret[-1]]: ret = ret[:-1] ret += ch return ret