【leetcode】316. Remove Duplicate Letters

题目如下：

Given a string which contains only lowercase letters, remove duplicate letters so that every letter appear once and only once. You must make sure your result is the smallest in lexicographical order among all possible results.

Example 1:

Input: "bcabc"
Output: "abc"

Example 2:

Input: "cbacdcbc"
Output: "acdb"

解题思路：因为要求字典序最小，所以先从'a'开始，找出第一个出现的'a'，如果在这个'a'后面其余的字符至少会出现一次，则表示满足条件；如果'a'不满足这个条件，则继续判断'b'，直到找出第一个满足条件并且字典序最小的字符。找到这个字典序最小的字符(假设为i)后，那么表示这个'i'可以保留，'i'之前的所有字符以及'i'之后其他的'i'都删除掉,在剩下的字符串中又继续从'a'开始，找到满足条件的除i以为最小的字典序字符。一直循环，直到所有不同字符都找到一个满足这个条件的即可。

代码如下：

class Solution(object):
    def removeDuplicateLetters(self, s):
        """
        :type s: str
        :rtype: str
        """
        dic = {}
        for i,v in enumerate(s):
            dic[v] = dic.setdefault(v,[]) + [i]
        res = ''
        origin_len = len(dic)

        while len(res) < origin_len:
            #print dic
            for (char) in range(ord('a'),ord('a')+26):
                char = chr(char)
                if char not in dic:
                    continue
                flag = True
                for key in dic.iterkeys():
                    if char == key:
                        continue
                    elif dic[char][0] > dic[key][-1]:
                        flag = False
                        break
                if flag:
                    res += char
                    inx = dic[char][0]
                    del dic[char]

                    import bisect
                    for key in dic.iterkeys():
                        del_inx = bisect.bisect_left(dic[key],inx)
                        dic[key] = dic[key][del_inx:]
                    break
        return res