python leetcode 784. Letter Case Permutation

Given a string S, we can transform every letter individually to be lowercase or uppercase to create another string.  Return a list of all possible strings we could create.

列举出字符串中在字母有大小写两种情况下的所有可能组合。

Examples:
Input: S = "a1b2"
Output: ["a1b2", "a1B2", "A1b2", "A1B2"]

Input: S = "3z4"
Output: ["3z4", "3Z4"]

Input: S = "12345"
Output: ["12345"]

Note:

• S will be a string with length at most 12.

• S will consist only of letters or digits.

class Solution:
    def letterCasePermutation(self, S):
        """
        :type S: str
        :rtype: List[str]
        """
        cur_s = [S]
        for i in range(len(S)):
            next_s = []
            for s in cur_s:
                if s[i].isdigit():
                    next_s.append(s)
                else:
                    next_s.append(s[0:i] + s[i].lower() + s[i+1:])
                    next_s.append(s[0:i] + s[i].upper() + s[i+1:])
            cur_s = next_s
        return cur_s